WAEC Mathematics 1997 past questions and answers

57 questions. Try each one, then open the answer to check yourself and read the explanation.

Question 1

Fractions, Decimals and Approximation

Express (0.0425 ÷ 2.5) as a fraction

  1. A.17/10,000
  2. B.17/1,000
  3. C.17/250
  4. D.17/100
  5. E.17/10
Show the answer and explanation

Answer: B (17/1,000)

Explanation

0.0425 ÷ 2.5 = 0.017, and 0.017 = 17/1000.

Open this question

Question 2

Logarithms

What is log10 (6.95 × 10−3)?

  1. A.0.8420
  2. B.3.8420
  3. C.3̄.8420
  4. D.3.9777
  5. E.3̄.9777
Show the answer and explanation

Answer: C (3̄.8420)

Explanation

log 6.95 = 0.8420 and the power 10−3 gives characteristic −3, so the log is 3̄.8420.

Open this question

Question 3

Sets

If P = {3, 5, 6} and Q = {4, 5, 6} then P∩Q equals

  1. A.{3, 4}
  2. B.{4, 5}
  3. C.{4, 6}
  4. D.{5, 6}
  5. E.{3, 6}
Show the answer and explanation

Answer: D ({5, 6})

Explanation

P∩Q holds the elements in both sets: 5 and 6.

Open this question

Question 4

Sets

A and B are two sets. The number of elements in A∪B is 49, the number in A is 22 and number in B is 34. How many elements are in A∩B?

  1. A.105
  2. B.27
  3. C.15
  4. D.12
  5. E.7
Show the answer and explanation

Answer: E (7)

Explanation

n(A∩B) = n(A) + n(B) − n(A∪B) = 22 + 34 − 49 = 7.

Open this question

Question 5

Ratio, Percentages and Rates

A student found the approximate value of 0.02548 correct to two places of decimal instead of two significant figures. Find the percentage error.

  1. A.0%
  2. B.13 1/3%
  3. C.15 5/13%
  4. D.16 2/3%
  5. E.20%
Show the answer and explanation

Answer: E (20%)

Explanation

To 2 d.p. the student got 0.03; the correct 2 s.f. value is 0.025. Error = 0.005, and 0.005/0.025 × 100% = 20%.

Open this question

Question 6

Logarithms

Simplify log39 + log315 − log35

  1. A.log319
  2. B.log3…
  3. C.3
  4. D.1
  5. E.0
Show the answer and explanation

Answer: C (3)

Explanation

Combine the logs: log3(9 × 15 ÷ 5) = log327 = 3.

Open this question

Question 7

Linear and Simultaneous Equations

Solve the equation: 2/3 (x + 5) = 1/4(5x − 3)

  1. A.1 1/7
  2. B.1 8/23
  3. C.3
  4. D.4 3/7
  5. E.7
Show the answer and explanation

Answer: E (7)

Explanation

Multiply both sides by 12: 8(x + 5) = 3(5x − 3), so 8x + 40 = 15x − 9, giving 7x = 49 and x = 7.

Open this question

Question 9

Linear and Simultaneous Equations

Solve the equations: 4x − y = 11, 5x + 2y = 4

  1. A.x = 6, y = 13
  2. B.x = −2, y = −3
  3. C.x = −2, y = 3
  4. D.x = 2, y = −3
  5. E.x = 2, y = 3
Show the answer and explanation

Answer: D (x = 2, y = −3)

Explanation

Double the first equation (8x − 2y = 22) and add the second: 13x = 26, so x = 2; then y = 4(2) − 11 = −3.

Open this question

Question 10

Algebraic Expressions

Make q the subject of the relation t = √(pq/r − r2q)

  1. A.q = rt2/(p − r3)
  2. B.q = t2/(p − r2)
  3. C.q = rt/(p − r3)
  4. D.q = (p − r3)/rt2
  5. E.q = rt2(p − r3)
Show the answer and explanation

Answer: A (q = rt2/(p − r3))

Explanation

Square: t2 = pq/r − r2q. Multiply by r: rt2 = q(p − r3), so q = rt2/(p − r3).

Open this question

Question 11

Quadratic Equations

One of the roots of the equation 6x2 = 5 − 7x is

  1. A.−1/2
  2. B.−1/3
  3. C.1/2
  4. D.1 2/3
  5. E.2 1/2
Show the answer and explanation

Answer: C (1/2)

Explanation

6x2 + 7x − 5 = 0 factorises as (2x − 1)(3x + 5) = 0, so x = 1/2 or x = −5/3.

Open this question

Question 12

Variation

P varies inversely as the square of W. When W = 4, P = 9. Find the value of P when W = 9

  1. A.729/16
  2. B.6
  3. C.4
  4. D.16/9
  5. E.4/3
Show the answer and explanation

Answer: D (16/9)

Explanation

P = k/W2; 9 = k/16 gives k = 144. When W = 9, P = 144/81 = 16/9.

Open this question

Question 14

Triangles and Quadrilaterals

In the triangle XYZ, XM is the altitude from X to YZ. XY = 13cm, XZ = 15cm and YM = 5cm. Find the length of YZ

Diagram for this question
  1. A.9cm
  2. B.12cm
  3. C.14cm
  4. D.19.85cm
  5. E.23cm
Show the answer and explanation

Answer: C (14cm)

Explanation

XM = √(132 − 52) = 12 cm, so MZ = √(152 − 122) = 9 cm. YZ = 5 + 9 = 14 cm.

Open this question

Question 15

Latitude and Longitude

Two points P and Q are on longitude 67°W. Their latitudes differ by 90°. Calculate their distance apart in terms of π. (Take radius of the earth = 6400km).

  1. A.6400πkm
  2. B.6400km/π
  3. C.3200πkm
  4. D.3200km
  5. E.3200km/π
Show the answer and explanation

Answer: C (3200πkm)

Explanation

On the same longitude the points lie on a great circle: distance = 90/360 × 2π × 6400 = 3200π km.

Open this question

Question 16

Mensuration of Plane Shapes

The length of the two parallel sides of trapezium are 6cm and 10cm and the perpendicular distance between them is 5cm. Find the area of the trapezium.

  1. A.21cm2
  2. B.40cm2
  3. C.80cm2
  4. D.150cm2
  5. E.300cm2
Show the answer and explanation

Answer: B (40cm2)

Explanation

Area = ½ × (6 + 10) × 5 = 40 cm2.

Open this question

Question 17

Mensuration of Solids

Calculate the total surface area of a solid cone of slant height 15cm and base radius 8cm in terms of π.

  1. A.64πcm2
  2. B.120πcm2
  3. C.184πcm2
  4. D.200πcm2
  5. E.320πcm2
Show the answer and explanation

Answer: C (184πcm2)

Explanation

Total surface area = πr2 + πrl = π × 8 × (8 + 15) = 184π cm2.

Open this question

Question 18

Mensuration of Solids

The cross-section of a prism is a right-angled triangle 3cm by 4cm by 5cm. The height of the prism is 8cm. Calculate its volume

  1. A.48cm3
  2. B.60cm3
  3. C.96cm3
  4. D.120cm3
  5. E.240cm3
Show the answer and explanation

Answer: A (48cm3)

Explanation

The right angle is between the 3 cm and 4 cm sides, so the cross-section is ½ × 3 × 4 = 6 cm2. Volume = 6 × 8 = 48 cm3.

Open this question

Question 19

Mensuration of Plane Shapes

Find the radius of a circle in which an arc of length 44cm subtends angle 200° at the centre of the circle. {Take π = 22/7}

  1. A.3.9cm
  2. B.12.6cm
  3. C.25.2cm
  4. D.38.4cm
  5. E.124.5cm
Show the answer and explanation

Answer: B (12.6cm)

Explanation

Arc = θ/360 × 2πr, so 44 = 200/360 × 2 × 22/7 × r, giving r = 12.6 cm.

Open this question

Question 20

Trigonometry

The diagram below is a triangle XYZ whose area is 23.5cm2. XY = 10cm and YZ = 8cm, calculate θ, correct to the nearest degree.

Diagram for this question
  1. A.34°
  2. B.35°
  3. C.36°
  4. D.37°
  5. E.40°
Show the answer and explanation

Answer: C (36°)

Explanation

Area = ½ × 10 × 8 × sin θ = 23.5, so sin θ = 0.5875 and θ ≈ 36°.

Open this question

Question 21

Constructions

Which of the following is not necessarily sufficient for the construction of a triangle?

  1. A.Three sides
  2. B.Two sides and an included angle
  3. C.Two sides and a right angle
  4. D.Two sides and an angle
  5. E.Two angles and a side.
Show the answer and explanation

Answer: D (Two sides and an angle)

Explanation

Two sides with an angle that is not between them can give two different triangles (the ambiguous case), so it does not always fix one triangle.

Open this question

Question 22

Triangles and Quadrilaterals

In the diagram, EF//QR, PE = 2cm, EQ = 4cm and FR = 6cm. Find x.

Diagram for this question
  1. A.2cm
  2. B.3cm
  3. C.4cm
  4. D.5cm
  5. E.6cm
Show the answer and explanation

Answer: B (3cm)

Explanation

A line parallel to one side divides the other sides in the same ratio: PE/EQ = PF/FR, so 2/4 = x/6 and x = 3 cm.

Open this question

Question 23

Angles and Polygons

Which of the following statements is/are true when two straight lines intersect?
I. Adjacent angles are equal
II. Vertically opposite angles are equal
III. Adjacent angles are supplementary

  1. A.II only
  2. B.I and II only
  3. C.I and III only
  4. D.II and III only
  5. E.I, II and III.
Show the answer and explanation

Answer: D (II and III only)

Explanation

Vertically opposite angles are always equal and adjacent angles on a straight line add to 180°; adjacent angles are equal only when the lines are perpendicular.

Open this question

Question 24

Angles and Polygons

PQRS and GHRS are parallelograms on the same base SR and between the same parallel straight line PH and SR. Which of the following is true?

Diagram for this question
  1. A.ΔPGS = ΔQHR
  2. B.Parallelogram PQRS = ΔQHR
  3. C.Trapezium PQXS = ΔGHR
  4. D.ΔPGS = Parallelogram GHRS
  5. E.Parallelogram PQRS = Parallelogram GHRS.
Show the answer and explanation

Answer: E (Parallelogram PQRS = Parallelogram GHRS.)

Explanation

Parallelograms on the same base and between the same parallels have equal areas.

Open this question

Question 25

Triangles and Quadrilaterals

From the diagram below, which of the following is true?

Diagram for this question
  1. A.a + b + x = 180
  2. B.a = b + x
  3. C.a − b = 180 − x
  4. D.a + b = x + 180
  5. E.a − b = x − 180.
Show the answer and explanation

Answer: D (a + b = x + 180)

Explanation

The top interior angle is 180 − a. The exterior angle b equals the sum of the two opposite interior angles: b = x + (180 − a), so a + b = x + 180.

Open this question

Question 26

Angles and Polygons

The interior angles of a pentagon are 126°, 114°, y, 92° and 83°. Find the value of y.

  1. A.85°
  2. B.95°
  3. C.105°
  4. D.115°
  5. E.125°.
Show the answer and explanation

Answer: E (125°.)

Explanation

Angles of a pentagon add to (5 − 2) × 180° = 540°. y = 540 − (126 + 114 + 92 + 83) = 125°.

Open this question

Question 27

Inequalities

Solve the inequality 3x − 8 ≥ 5x

  1. A.x ≤ 4
  2. B.x ≥ 1
  3. C.x ≤ −4
  4. D.x ≤ −1
  5. E.x < 2.
Show the answer and explanation

Answer: C (x ≤ −4)

Explanation

3x − 5x ≥ 8 gives −2x ≥ 8. Dividing by −2 reverses the sign: x ≤ −4.

Open this question

Question 28

Fractions, Decimals and Approximation

Divide 3̄.6721 by 4

  1. A.0.9180
  2. B.1̄.4180
  3. C.1.1680
  4. D.1̄.1680
  5. E.1.9180.
Show the answer and explanation

Answer: B (1̄.4180)

Explanation

Write 3̄.6721 as 4̄ + 1.6721. Dividing by 4 gives 1̄ + 0.4180 = 1̄.4180.

Open this question

Question 29

Progressions

The eleventh term of an A.P. is 25 and its first term is −3. Find its common difference.

  1. A.1 9/10
  2. B.2 1/5
  3. C.2 3/11
  4. D.2 1/2
  5. E.2 4/5.
Show the answer and explanation

Answer: E (2 4/5.)

Explanation

T11 = a + 10d, so −3 + 10d = 25, 10d = 28, d = 2 4/5.

Open this question

Question 30

Progressions

Find the common ratio in the G.P: log 3, log 9, log 81……….

  1. A.1
  2. B.2
  3. C.3
  4. D.6
  5. E.9.
Show the answer and explanation

Answer: B (2)

Explanation

log 9 = 2 log 3 and log 81 = 4 log 3, so each term is twice the one before: r = 2.

Open this question

Question 31

Indices

Find the value of t for which 64/27 = (3/4)t−1

  1. A.−4
  2. B.−2
  3. C.1 1/3
  4. D.2
  5. E.4.
Show the answer and explanation

Answer: B (−2)

Explanation

64/27 = (4/3)3 = (3/4)−3, so t − 1 = −3 and t = −2.

Open this question

Question 32

Circle Theorems

O is the centre of the circle PQRS. PR and QS intersect at T. POR is a diameter, ∠PQT = 42° and ∠QTR = 64°. Find ∠QRT

Diagram for this question
  1. A.68°
  2. B.64°
  3. C.42°
  4. D.32°
  5. E.22°.
Show the answer and explanation

Answer: A (68°)

Explanation

PR is a diameter, so ∠PQR = 90° and ∠TQR = 90 − 42 = 48°. In triangle QTR, ∠QRT = 180 − 64 − 48 = 68°.

Open this question

Question 33

Circle Theorems

TR is the tangent to the circle PQR with centre O. Find the size of ∠PRT

Diagram for this question
  1. A.44°
  2. B.46°
  3. C.60°
  4. D.74°
  5. E.118°.
Show the answer and explanation

Answer: C (60°)

Explanation

∠QOR = 2 × 74° = 148°, so in isosceles triangle OQR ∠OQR = 16°. ∠PQR = 44 + 16 = 60°, and the tangent–chord angle ∠PRT equals ∠PQR in the alternate segment: 60°.

Open this question

Question 34

Triangles and Quadrilaterals

Two angles of a triangle are 41° and 49°. Which of the following is true? It is

  1. A.an equilateral triangle
  2. B.an isosceles triangle
  3. C.a triangle with an obtuse angle
  4. D.a right-angled triangle
  5. E.a right-angled isosceles triangle.
Show the answer and explanation

Answer: D (a right-angled triangle)

Explanation

The third angle is 180 − 41 − 49 = 90°, and the other two angles differ, so it is right-angled but not isosceles.

Open this question

Question 35

Logarithms

Given that 1/2 log10 P = 1, find the value of P

  1. A.104
  2. B.103
  3. C.102
  4. D.101
  5. E.10−1
Show the answer and explanation

Answer: C (102)

Explanation

log10 P = 2, so P = 102.

Open this question

Question 36

Triangles and Quadrilaterals

If two triangles are similar, which of the following is true? Their

  1. A.corresponding sides are equal
  2. B.corresponding angles are equal
  3. C.corresponding altitudes are equal
  4. D.areas are equal
  5. E.perimeters are equal.
Show the answer and explanation

Answer: B (corresponding angles are equal)

Explanation

Similar triangles have the same shape: their corresponding angles are equal, while their sides are only in the same ratio.

Open this question

Question 37

Triangles and Quadrilaterals

Which of the following is not necessarily true of a rectangle?

  1. A.The diagonals are equal
  2. B.The diagonals bisect each other
  3. C.The diagonals are perpendicular
  4. D.Each diagonal divides the area of the rectangle equally
  5. E.Each diagonal divides the rectangle into two congruent triangles.
Show the answer and explanation

Answer: C (The diagonals are perpendicular)

Explanation

A rectangle's diagonals cross at right angles only when it is a square.

Open this question

Question 38

Circle Theorems

A chord is 5cm from the centre of a circle of diameter 26cm. Find the length of the chord

  1. A.16cm
  2. B.18cm
  3. C.24cm
  4. D.25.51cm
  5. E.51.1cm.
Show the answer and explanation

Answer: C (24cm)

Explanation

Radius = 13 cm. Half the chord = √(132 − 52) = 12 cm, so the chord is 24 cm.

Open this question

Question 39

Trigonometry

The angle of elevation of a point T on a tower from a point U on the horizontal ground is 30°. If TU = 54m, how high is T above the horizontal ground?

  1. A.108m
  2. B.72m
  3. C.46.3m
  4. D.31.2m
  5. E.27m.
Show the answer and explanation

Answer: E (27m.)

Explanation

TU is the slanting line of sight, so height = 54 × sin 30° = 27 m.

Open this question

Question 40

Bearings

Express the true bearing of 250° as a compass bearing

  1. A.N20°E
  2. B.S20°E
  3. C.N20°W
  4. D.S70°W
  5. E.S70°E.
Show the answer and explanation

Answer: D (S70°W)

Explanation

250° lies between south (180°) and west (270°); it is 70° west of south: S70°W.

Open this question

Question 42

Trigonometry

Cos 65° has the same value as

  1. A.Sin 65°
  2. B.Cos 25°
  3. C.Cos 115°
  4. D.Cos 205°
  5. E.Cos 295°
Show the answer and explanation

Answer: E (Cos 295°)

Explanation

Cosine is positive in the fourth quadrant and cos(360° − θ) = cos θ, so cos 295° = cos 65°.

Open this question

Question 43

Trigonometry

Two sides of a triangle are perpendicular. If the two sides are 8cm and 6cm, calculate correct to the nearest degree, the smallest angle of the triangle.

  1. A.35°
  2. B.36°
  3. C.37°
  4. D.38°
  5. E.53°.
Show the answer and explanation

Answer: C (37°)

Explanation

The smallest angle is opposite the 6 cm side: tan θ = 6/8 = 0.75, so θ ≈ 36.9° ≈ 37°.

Open this question

Question 44

Data Presentation

The graph is the cumulative frequency curve for the weight distribution of 100 workers in a factory. Which of the points P, Q, R, S and T indicates the median weight?

Diagram for this question
  1. A.T
  2. B.S
  3. C.R
  4. D.Q
  5. E.P.
Show the answer and explanation

Answer: D (Q)

Explanation

The median is the weight at half the total frequency (50). Go across from 50 to the curve at R and down to the weight axis at Q.

Open this question

Question 46

Measures of Dispersion

Calculate the standard deviation of the numbers 2, 5, 6, 4 and 8

  1. A.2
  2. B.4
  3. C.6
  4. D.7
  5. E.20.
Show the answer and explanation

Answer: A (2)

Explanation

Mean = 25/5 = 5. Squared deviations: 9, 0, 1, 1, 9, total 20. Variance = 20/5 = 4, so the standard deviation is 2.

Open this question

Question 48

Probability

What is the probability of having an odd number in a single throw of a fair die with the faces numbered 1,2,3,4,5 and 6?

  1. A.5/6
  2. B.2/3
  3. C.1/2
  4. D.1/3
  5. E.1/6.
Show the answer and explanation

Answer: C (1/2)

Explanation

Three of the six faces (1, 3, 5) are odd: 3/6 = 1/2.

Open this question

Question 49

Probability

A bag contains red, black and green identical balls. A ball is picked and replaced. The table shows the result of 100 trials. Find the experimental probability of picking a green ball.
Colour of ball: Red, Black, Green
No. of occurrences: 54, 30, 16

  1. A.16
  2. B.21/25
  3. C.1/3
  4. D.4/21
  5. E.4/25.
Show the answer and explanation

Answer: E (4/25.)

Explanation

Green came up 16 times in 100 trials: 16/100 = 4/25.

Open this question

Question 50

Probability

A bag contains 3 white, 6 red 5 blue identical balls. A ball is picked at random from the bag. What is the probability that it is either white or blue?

  1. A.9/14
  2. B.4/7
  3. C.3/7
  4. D.5/14
  5. E.3/14.
Show the answer and explanation

Answer: B (4/7)

Explanation

There are 14 balls, 3 + 5 = 8 of them white or blue: 8/14 = 4/7.

Open this question

Theory question 101

Number Bases

(a) Copy and complete the binary multiplication table
× | 10 | 11 | 100 | 101
10 | 100 | – | 1000 | –
11 | 110 | – | 1100 | –
100 | – | – | 10000 | 10100
(b) Convert 11.011two to a number in base ten
(c) Simplify (9.6 × 1018)/(0.24 × 105) and express your answer in the form P × 10m where 1 < P < 10 and m is an integer.

Show the model answer
Model answer

(a) Missing entries: 10 × 11 = 110; 10 × 101 = 1010; 11 × 11 = 1001; 11 × 101 = 1111; 100 × 10 = 1000; 100 × 11 = 1100.
(b) 11.011two = 1×2 + 1×1 + 0×½ + 1×¼ + 1×⅛ = 2 + 1 + 0.25 + 0.125 = 3.375.
(c) 9.6/0.24 = 40, so the value is 40 × 1018−5 = 40 × 1013 = 4 × 1014.

Explanation

Check each binary product by converting to base ten (e.g. 3 × 3 = 9 = 1001two).

Open this question

Theory question 102

Progressions

(a) The 6th term of an A.P. is 35 and the 13th term is 77. Find the 20th term.
(b) The Venn diagram represents three subsets P, Q and R of the universal set U. Copy the Venn diagram. Shade and indicate the regions represented by: (i) P∩Q'∩R (ii) P'∩Q∩R'.

Diagram for this question
Show the model answer
Model answer

(a) a + 5d = 35 and a + 12d = 77. Subtract: 7d = 42, d = 6; a = 35 − 30 = 5.
T20 = a + 19d = 5 + 114 = 119.
(b)(i) P∩Q'∩R: the part common to P and R that lies outside Q.
(ii) P'∩Q∩R': the part of Q that is outside both P and R (Q only).

Explanation

Find d first by subtracting the two term equations.

Open this question

Theory question 103

Trigonometry

(a) Given that sin x = 5/13, 0° ≤ x ≤ 90°, find (cos x − 2 sin x)/(2 tan x)
(b) The diagram represents the vertical cross-section of a mountain with height NQ standing on a horizontal ground PRN. If the angles of elevation of the top Q of the mountain from P and R are 30° and 70° respectively and PR = 500m, calculate, correct to 3 significant figures: (i) |QP|; (ii) the height of the mountain.

Diagram for this question
Show the model answer
Model answer

(a) Opposite = 5, hypotenuse = 13, so adjacent = 12: cos x = 12/13, tan x = 5/12.
(cos x − 2 sin x)/(2 tan x) = (12/13 − 10/13) ÷ (10/12) = 2/13 × 12/10 = 12/65.
(b) In triangle PQR, ∠PRQ = 110°, so ∠PQR = 180 − 30 − 110 = 40°.
(i) Sine rule: QP/sin 110° = 500/sin 40°, QP = 500 × 0.9397/0.6428 ≈ 731 m.
(ii) Height NQ = QP sin 30° = 730.9 × 0.5 ≈ 365 m.

Explanation

Use the exterior angle at R to find the angle at Q, then the sine rule.

Open this question

Theory question 104

Data Presentation

The table below shows how a company's sales manager spent his 1995 annual salary.
Food – 30%
Rent – 18%
Car maintenance – 25%
Savings – 12%
Taxes – 5%
Others – 10%
(a) Represent this information on a pie chart
(b) Find his savings at the end of the year if his annual salary was N60,000.00

Show the model answer
Model answer

(a) Angle = percentage × 3.6°: Food 108°, Rent 64.8°, Car maintenance 90°, Savings 43.2°, Taxes 18°, Others 36° (total 360°). Draw the sectors with a protractor and label each.
(b) Savings = 12% of N60,000 = N7,200.00.

Explanation

Each 1% of the whole is 3.6° of the circle.

Open this question

Theory question 105

Algebraic Expressions

(a) Given that (5y − x)/(8y + 3x) = 1/5, find the value of x/y to two decimal places.
(b) If 3 is a root of the quadratic equation x2 + bx − 15 = 0, determine the value of b. Find the other root.

Show the model answer
Model answer

(a) 5(5y − x) = 8y + 3x, so 25y − 5x = 8y + 3x, 17y = 8x and x/y = 17/8 = 2.125 ≈ 2.13.
(b) 32 + 3b − 15 = 0, so 3b = 6 and b = 2.
x2 + 2x − 15 = (x − 3)(x + 5) = 0, so the other root is −5.

Explanation

Cross-multiply, then collect the x and y terms on opposite sides.

Open this question

Theory question 106

Logarithms

(a) Use logarithm tables to evaluate (15.05 × √0.00695)/(6.95 × 102), correct to 3 significant figures.
(b) The first 5 students to arrive in a school on a Monday morning were 2 boys and 3 girls. Of these, two were chosen at random for an assignment. Find the probability that: (i) both were boys; (ii) the two were of different sexes.

Show the model answer
Model answer

(a) log 15.05 = 1.1775; log 0.00695 = 3̄.8420, half of it = 2̄.9210 (write 4̄ + 1.8420, divide by 2); log 695 = 2.8420.
Numerator: 1.1775 + 2̄.9210 = 0.0985. Subtract 2.8420: 3̄.2565.
Antilog = 0.001806 ≈ 0.00181.
(b) Two are chosen without replacement.
(i) P(both boys) = 2/5 × 1/4 = 1/10.
(ii) P(different sexes) = 2/5 × 3/4 + 3/5 × 2/4 = 6/20 + 6/20 = 3/5.

Explanation

Two different students are chosen, so the second pick is from 4, not 5.

Open this question

Theory question 107

Constructions

(a) Using a ruler and a pair of compasses only, construct ΔABC in which |AB| = 7cm, |BC| = 5cm and ∠ABC = 75°. Measure AC.
(b) In (a) above, locate by construction, a point D such that CD is parallel to AB and D is equidistant from points A and C. Measure ∠BAD.

Show the model answer
Model answer

(a) Draw BC = 5 cm. At B construct 75° (60° plus half of the 30° between 60° and 90°). Mark A on this arm with BA = 7 cm and join AC.
AC ≈ 7.5 cm (by calculation √(49 + 25 − 70 cos 75°) ≈ 7.48 cm).
(b) Through C construct a line parallel to AB (copy ∠ABC at C, or use parallelogram construction). Construct the perpendicular bisector of AC; D is where it meets the parallel line.
∠BAD ≈ 80° (accept 79°–81°).

Explanation

D must lie on two loci at once: the line through C parallel to AB and the perpendicular bisector of AC.

Open this question

Theory question 108

Logarithms

(a) Solve the simultaneous equations:
log10x + log10y = 4
log10x + 2 log10y = 3
(b) The time t taken to buy fuel at a petrol station varies directly as the number of vehicles V on queue and jointly varies inversely as the number of pumps P available in the station. In a station with 5 pumps, it took 10 minutes to fuel 20 vehicles. Find (i) the relationship between t, P and V; (ii) the time it will take to fuel 50 vehicles in the station with 2 pumps; (iii) the number of pumps required to fuel 40 vehicles in 20 minutes.

Show the model answer
Model answer

(a) Subtract the first equation from the second: log10y = −1, so y = 1/10.
Then log10x = 4 − (−1) = 5, so x = 100,000.
(b)(i) t = kV/P. 10 = k × 20/5 gives k = 5/2, so t = 5V/(2P).
(ii) t = 5 × 50/(2 × 2) = 62.5 minutes (1 hr 2½ min).
(iii) 20 = 5 × 40/(2P), so P = 200/40 = 5 pumps.

Explanation

Treat log x and log y as the unknowns and eliminate one of them.

Open this question

Theory question 109

Mensuration of Solids

The solid above is a cylinder surmounted by a hemispherical bowl. Calculate its: (a) total surface area; (b) Volume. (Take π = 22/7).

Diagram for this question
Show the model answer
Model answer

From the diagram the radius is 7 cm and the cylinder is 10 cm high.
(a) Curved surface of cylinder = 2πrh = 2 × 22/7 × 7 × 10 = 440 cm2.
Hemisphere = 2πr2 = 308 cm2. Base = πr2 = 154 cm2.
Total = 440 + 308 + 154 = 902 cm2.
(b) Volume = ⅔πr3 + πr2h = 718⅔ + 1540 = 2258⅔ ≈ 2258.7 cm3.

Explanation

Add the separate surfaces that are on the outside: cylinder wall, hemisphere and flat base.

Open this question

Theory question 110

Graphs of Functions

Above is the graph of the quadratic function y = ax2 + bx + c where a, b and c are constants. Using the graph, find: (a)(i) The scales on both axis; (ii) The equation of the line of symmetry of the curve; (iii) The roots of the quadratic equation ax2 + bx + c = 0. (b) Use the coordinates of points D, E and G to find the values of the constants a, b, and c and hence write down the quadratic function illustrated in the graph. (c) Find the greatest value of y within the range −3 ≤ x ≤ 5.

Diagram for this question
Show the model answer
Model answer

(a)(i) Read the scales from the grid: about 2 cm to 1 unit on the x-axis and 2 cm to 5 units on the y-axis.
(ii) The curve is lowest at about x = 1.25, so the line of symmetry is x = 1.25 (accept 1.2–1.3).
(iii) The curve cuts the x-axis at about x = 0.2 and x = 2.3.
(b) From the graph D = (0, 1), E = (1, −2), G = (3, 4).
D: c = 1. E: a + b + 1 = −2, so a + b = −3. G: 9a + 3b + 1 = 4, so 3a + b = 1.
Subtract: 2a = 4, a = 2, b = −5. y = 2x2 − 5x + 1.
(c) In −3 ≤ x ≤ 5 the greatest value is at x = −3: y = 18 + 15 + 1 = 34.

Explanation

Substitute each point into y = ax² + bx + c to get three equations.

Open this question

Theory question 111

Circle Theorems

(a) PQRST is a circle with centre C. PCS is a straight line, RS//QT, |QR| = |RS| and ∠QTS = 56°. Find (i) ∠SQT (ii) ∠PQT
(b) In the diagram, points B and C are on a horizontal plane and |BC| = 30cm. A and D are points vertically above B and C respectively. |DC| = 40cm and |AB| = 26cm. Calculate the angles of depression of: (i) B from D; (ii) A from D: correct to the nearest degree.

Diagram for this question
Show the model answer
Model answer

(a)(i) ∠QPS = ∠QTS = 56° (same segment). PQRS is cyclic, so ∠QRS = 180 − 56 = 124°.
Triangle QRS is isosceles (QR = RS), so ∠RSQ = (180 − 124)/2 = 28°.
RS ∥ QT, so ∠SQT = ∠RSQ = 28° (alternate angles).
(ii) PS is a diameter, so ∠PQS = 90°. ∠PQT = 90 − 28 = 62°.
(b)(i) The angle of depression of B from D equals ∠DBC: tan = 40/30, angle ≈ 53.1° ≈ 53°.
(ii) A is 40 − 26 = 14 cm below D over a horizontal 30 cm: tan = 14/30, angle ≈ 25.0° ≈ 25°.

Explanation

An angle of depression is measured down from the horizontal at the observer.

Open this question

Theory question 112

Data Presentation

The table below shows the mark distribution of candidates in an aptitude test for selection into the public service.
Mark (in %): 44–46, 47–49, 50–52, 53–55, 56–58, 59–61, 62–64, 65–67, 68–70, 71–73
Frequency: 2, 5, 11, 20, 26, 42, 46, 36, 9, 3
(a) Make a cumulative frequency for the distribution.
(b) Draw the cumulative frequency curve.
(c) From your graph estimate the median mark.
(d) The cut-off mark was 63%. What percentage of the candidates was selected?

Show the model answer
Model answer

(a) Upper class boundaries 46.5, 49.5, 52.5, 55.5, 58.5, 61.5, 64.5, 67.5, 70.5, 73.5 with cumulative frequencies 2, 7, 18, 38, 64, 106, 152, 188, 197, 200.
(b) Plot cumulative frequency against upper class boundary (start at (43.5, 0)) and join with a smooth S-shaped curve.
(c) Median at the 100th candidate: 58.5 + (100 − 64)/42 × 3 ≈ 61% (from the graph about 61).
(d) Candidates below 62.5: about 106 + 46/3 ≈ 121, so about 79 scored 63 or more: 79/200 × 100 ≈ 39% (accept 36–40% read from the graph).

Explanation

Always plot cumulative frequency against the upper class boundary, not the class mark.

Open this question