p(b)= 2/5+(3/5)* y/(n- 1)≤2/5+3/5 * 1/2 = 7/ 10

In this sentence:

There are two ways to touch two balls:

1, the first touch is a black ball, regardless of the color of the second touch.

2. If you find the black ball first, the second one must be the black ball.

The probability of the first pair is 2/5* 1=2/5.

The probability of the second one is 3/5* and the probability of finding y black balls from the remaining n- 1 balls is 3/5 * y/(n- 1).