Question 20

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Jambmaths question: 

A bag contains 5 blacks, 4 white and x red marble. If the probability of picking a red marble is $\tfrac{2}{5}$, find the value of x 

Option A: 

6

Option B: 

10

Option C: 

4

Option D: 

8

Jamb Maths Solution: 

$\begin{align}  & \text{Total number of marbles}=5+4+x=9+x \\ & \Pr (Red)=\tfrac{2}{5} \\ & \frac{x}{9+x}=\frac{2}{5} \\ & 5x=18+2x \\ & 3x=18 \\ & x=6 \\\end{align}$

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