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JAMB Mathematics 1978 Question 27

Question 27

Progressions

In a geometric progression the first term is 153 and the sixth term is 17/27. The sum of the first four terms is

  1. A.860/3
  2. B.680/3
  3. C.608/3
  4. D.806/3
  5. E.680/27
Show the answer and explanation

Answer: B (680/3)

Explanation

153r⁵ = 17/27 gives r⁵ = 1/243, so r = 1/3. S₄ = 153(1 − 1/81)/(1 − 1/3) = 153 × 80/81 × 3/2 = 680/3.