Theory question 104
Probability(a) A box contains 40 identical discs which are either red or white. If the probability of picking a red disc is ¼, calculate the number of: (i) white discs; (ii) red discs that should be added such that the probability of picking a red disc will be ⅓.
(b) A salesman bought some plates at ₦50.00 each. If he sold all of them for ₦600.00 and made a profit of 20% on the transaction, how many plates did he buy?
Show the model answer
Model answer
(a) Red = ¼ × 40 = 10.
(i) White = 40 − 10 = 30.
(ii) (10 + r)/(40 + r) = ⅓ → 30 + 3r = 40 + r → r = 5 red discs.
(b) ₦600 is 120% of the cost, so cost = 600 × 100/120 = ₦500. Number of plates = 500 ÷ 50 = 10.
Explanation
Adding red discs increases both the red count and the total.