Question

In a box, there are 9 blue, 6 white and some black stones. A stone is randomly selected and the probability that the stone is black is ¼. Find the total number of stones in the box?

Answer 1

Number of blue stones=9
Number of white stones=6
Let number of black stones be x
So,
Probability of getting a black stone=
Number of favorable outcomes/Total number of outcomes
= x/9+6+x
=x/15+x

According to the question ,
Probability of getting black stone=1/4
So,
x/15+x=1/4
4x=15+x
4x-x=15
3x=15
x=15/3
x=5
Total number of stones=9+6+5=20 stones
So, there are 20 stones in the box.