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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