Calculate the probability of taking 0 white, 1 white and 2 white in the first two times, pa 1, pa2 and pa3.

Then the topic becomes

0 white example: 4 balls, 2 black balls, 2 white balls, and the expected Ea 1 of white balls.

1 white: 4 balls, 3 black 1 white, take the expected Ea2 of the white ball.

2 White example: 4 balls, 4 black balls and 0 white balls, and the expected Ea3=0 for white balls is 0.

ea = pa 1×ea 1+pa2×Ea2+pa3×Ea3