# CBSE 2017 Board Solution

10th standard Math - Set 3 - Q12. Probability

#### Video Explanation

Bag contains 15 white balls
Let the bag contain ‘n’ black balls
Total number of balls = (15 + n)

Probability of drawing a black ball = $$frac{\text{number of black balls}}{\text{total number of balls}}$ = $\frac{n}{$15 + n$})

Probability of drawing a white ball = $$frac{\text{number of white balls}}{\text{total number of balls}}$ = $\frac{15}{$15 + n$})

Probability of drawing a black ball = 3(Probability of drawing a white ball)
i.e., $$frac{n}{$15 + n$}) = 3$frac{15}{15 + n}$)
or n = 45

The number of black balls = 45.

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