WAEC 2020 · Paper 1 · Q17

Calculate the probability that the product of two numbers selected at random with replacement from the set $\{-5, -2, 4, 8\}$ is positive.

Worked solution (try it first)

A product is positive when both numbers have the same sign.
Two of the four numbers are negative and two positive, so each sign has probability $\frac{1}{2}$ on each pick.
Both negative: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$ .
Both positive: also $\frac{1}{4}$ .
So $P(\text{positive}) = \frac{1}{4} + \frac{1}{4}$
$= \frac{1}{2}$ , option B.