WAEC 2020 · Paper 2 · Q13

Class interval	1–10	11–20	21–30	31–40	41–50	51–60	61–70
Frequency	8	12	10	8	3	2	7

The table shows the frequency distribution of scores obtained in a Mathematics test.

Worked solution (try it first)

Class marks $5.5, 15.5, \ldots, 65.5$ with $\sum f = 50$ and $\sum fx = 1475$ , so the mean is $29.5$ .
Distances from 29.5: $24, 14, 4, 6, 16, 26, 36$ .
$\sum f|x - \bar x| = 192 + 168 + 40 + 48 + 48 + 52 + 252$
$= 800$ .
Mean deviation $= \dfrac{800}{50} = 16$ .