Ball pocket probability

There are seven red balls and 13 blue balls in the pocket. How many blue balls do we need to add to the pocket so that the probability of removing the red ball is 0.2?

Final Answer:

x =  15

Step-by-step explanation:

$$\begin{gathered} c=7 \\ m=13 \\ p=0.2=\frac{c}{c+m+x} \\ 0.2\cdot (c+m+x)=c \\ 0.2\cdot (7+13+x)=7 \\ 0.2x=3 \\ x=\frac{3}{0.2}=15 \\ x=15 \end{gathered}$$

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