Question

The mean of 25 observations is 36. Out of these observations, the mean of first 13 is 32 and that of the last 13 is 40. The 13th observations is
(a) 23
(b) 36
(c) 38
(d) 40

Answer 1

(b) 36

Mean of 25 observations = 36
Sum of 25 observations = $36\times 25=900$
Mean of the first 13 observations = 35
Sum of the first 13 observations = 32$\times$13 = 416
Mean of the last 13 observations = 40
Sum of the last 13 observations = 40$\times$13 = 520
∴ 13th observation = (Sum of the first 13 observations + Sum of the last 13 observations) $-$ Sum of 25 observations
= 416 + 520 $-$ 900
= 36