WAEC 2017 · Paper 2 · Q6

Age (years)	15–18	19–22	23–26	27–30	31–34	35–38
Frequency	40	33	25	10	8	4

The table shows a frequency distribution.

Worked solution (try it first)

Class marks $16.5, 20.5, 24.5, 28.5, 32.5, 36.5$ and $d = x - 24.5$ : $-8, -4, 0, 4, 8, 12$ .
$\sum fd = -320 - 132 + 0 + 40 + 64 + 48$
$= -300$ and $\sum f = 120$ .
Mean $= 24.5 + \dfrac{-300}{120}$
$= 24.5 - 2.5$
$= 22$ .