Lang Jun hunting English team to answer your questions ~

Let the black ball be X.

Then the probability that there are no black balls in these five times is p = (64-x)/64 * (63-x)/64 * ... * (60-x)/64.

Conclusion needs P.

When x=9, p is about 0.46.

When x=8, p is about 0.5 1.

Therefore, there must be at least nine black balls to ensure the conclusion is established.