Let A1, A2, A3, , A39 be 39 arithmetic means between the numbers 59 and 159. Then the mean of A25, A28, A31 and A36 is equal to :
- 129
- 136
- 131.50
- 134
Answer
(4) 134
Solution
59, A1, A2, , A39, 159 form an A.P. of 41 terms, so
159 = 59 + 40d d = 52
Ak = 59 + kd = 59 + 5k2
Hence
Mean = A25 + A28 + A31 + A364 = 59 + 52 25 + 28 + 31 + 364
∴ Mean = 59 + 52 30 = 59 + 75 = 134
Answer: (4)