WAEC 2019 · Paper 2 · Q5

Probability

Three red balls, five green balls and a number of blue balls are put together in a sack. One ball is picked at random from the sack. If the probability of picking a red ball is $\frac16$ , find:

(a)
the number of blue balls in the sack;
(b)
the probability of picking a green ball.

Worked solution (try it first)

(a)

Let there be $b$ blue balls.
Then there are $3 + 5 + b = 8 + b$ balls in the sack, and $P(\text{red}) = \frac{3}{8 + b} = \frac16$ .
Cross-multiply: $8 + b = 18$ , so $b = 10$ blue balls.

(b)

There are 18 balls, 5 of them green: $P(\text{green}) = \frac{5}{18}$ .

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