High school math. A high probability question for help! How to do the second question of 18 in the senior high school entrance examination? (Required answer: Highlight the number of basic events.

1) has x white balls.

The probability of touching the white ball for the first time is x/6, and the probability of touching the white ball for the second time is (x- 1)/5.

The probability of touching the white ball twice is x/6*(x- 1)/5=2/5.

To solve this equation: x=4 or x=-3 (rounding)

2) zeta =1,that is, the probability of getting the white ball for the first time =4/6=2/3.

, ξ=2, take out the black ball for the first time and the white ball for the second time. The probability is 2/6 * 4/5 = 4/ 15.

ξ=3, the first two times were all black balls, and the third time must be a white ball (only two black balls), and the probability = 2/6 *1/5 =115.

Mathematical expectation =1* 2/3+2 * 4/15+3 *115 = 7/5.