Question 41

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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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