JAMB 2003 · UME · Q42

The mean of the numbers 3, 6, 4, $x$ and 7 is 5. Find the standard deviation.

Worked solution (try it first)

The mean is 5, so the five numbers add up to 25: $20 + x = 25$ and $x = 5$ .
The squared deviations from 5 are 4, 1, 1, 0, 4, which add up to 10.
The variance is $\frac{10}{5} = 2$ , so the standard deviation is $\sqrt2$ , option D.