Let the number of black balls be $\mathit{x}$, then
Total number of balls $=30+\mathit{x}$
Thus, the probability of black balls $=\frac{\mathit{x}}{30+\mathit{x}}$
and the probability of white balls $=\frac{30}{30+\mathit{x}}$
Given : Probability of black ball

$\frac{\mathit{x}}{30+\mathit{x}}=\frac{2}{5}\times \frac{30}{\mathit{x}+30}$
$5\mathit{x}=60$
$\mathit{x}=12$
Hence, the number of black balls $=12$.