The probability of getting the white ball for the first time is 4/ 10.
The probability of getting the yellow ball again is 6/9.
Multiplication answer 4/ 15
5. Interpolation method
Five girls have six gaps.
Insert three boys.
That is, p (6 3) =120 species.
4.3 objectives
The probability that the green ball is zero is c (4 3)/c (6 3) =1/5.
The probability of having 1 green ball is c (4 2) c (21)/c (6 3) = 3/5.
The probability of having two green balls is c (41) c (22)/c (63) =1/5.
The expected value is 0 *1/5+1* 3/5+2 *1/5 =1.