A bag contains 15 white and some black balls.

The number of white balls in a bag = 15.
Let the number of black balls in that bag be x.
Then, the total number of balls in bag = 15+x.

Now, $\mathrm{P}($ black ball $)=\frac{x}{15+x}$ and $\mathrm{P}($ white ball $)=\frac{15}{15+x}$.

According to question, P(black ball) = 3 P(white ball)

$\Rightarrow \frac{x}{15+x}=3 \times \frac{15}{15+x}$

$\Rightarrow \frac{x}{15+x}=\frac{45}{15+x}$

$\Rightarrow x=45 .$

Hence, number of black balls in bag = x = 45.