Let the number of black balls be x, then
Total number of balls =30+x
Thus, the probability of black balls =‌

x

30+x

and the probability of white balls =‌

30

30+x

Given : Probability of black ball
‌=‌

2

5

×‌ probability of white ball ‌
‌

x

30+x

‌=‌

2

5

×‌

30

x+30

5x‌=60
x‌=12
Hence, the number of black balls =12.