Question

A bag contains 15 White and some black balls. If the probability of drawing a black ball from the bag is thrice that of drawing a white ball find the number of black balls in the bag.

Answer 1

Here,
No. of white balls = 15
Let, the no. of black balls be 'x'
Then, total no. of balls = 15 + x
Now,
P(drawing a white ball) = $\frac{15}{15+x}$
And, P(drawing a black ball) =
As per the question,

=> x = 315
=> x = 45
Therefore, there are 45 black balls in the bag.