WAEC 2020 · Paper 1 · Q35

The mean of the numbers 15, 21, 17, 26, 18 and 29 is 21. Calculate the standard deviation.

Worked solution (try it first)

The deviations from the mean 21 are $-6$ , 0, $-4$ , 5, $-3$ , 8.
Their squares are 36, 0, 16, 25, 9, 64, which add up to 150.
The variance is $\frac{150}{6} = 25$ , so the standard deviation is $\sqrt{25} = 5$ , option C.