Question

the mean of 25 observations is 18. the mean of first 12 is 14 and the mean of last twelve is 17. find the thirteenth result

Answer 1

$sum \: of \: 25 \: observations = 18 \times 25 = 450 \\ sum \: of \:1st \: 12 observations \ = 12 \times 14 = 168 \\ sum \: of \: last \: 12 \: observations = 12 \times 17 = 204 \\ 13th \: observation = 450 - (168 + 204) \\ = 450 - 372 \\ = 78$