waecmaths question:
A box contains black, white and red identical balls. The probability of picking a black ball at random from the box is $\tfrac{3}{10}$ and the probability of picking a white ball at random is $\tfrac{2}{5}$. If there are 30 balls in the box, how many of them are red?
Option A:
3
Option B:
7
Option C:
9
Option D:
12
waecmaths solution:
$\begin{align} & \frac{3}{10}+\frac{2}{5}+x=1 \\ & x=1-\frac{3}{10}-\frac{2}{5} \\ & x=\frac{3}{10} \\ & x=\Pr (R)=\frac{3}{10} \\ & \text{Number of red balls }=\frac{3}{10}\times 30=9 \\ &\text{There are 9 red balls} \\\end{align}$
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