Analysis: Under the same conditions, when a large number of repeated experiments are carried out, the frequency of random events gradually stabilizes near the probability. We can start with the proportional relationship and list the equations that require solutions.

Answer:

Solution: According to the meaning of the question, 3/a× 100%=20%,

Solution, a= 15.

So the answer is 15. Analysis: Because every ball is likely to be hit, we can use the probability formula to find out the probability of hitting a white ball and the probability that the ball is not a white ball, and list the equations to find out the relationship between m and n. 。

Arrange m+n = 8.

So the answer is: m+n = 8.