The probability of event a happening in one experiment is 2 / 3, and the probability of event a happening twice in four repeated independent experiments is?
C(4,2) (2/3)^2 * (1-2/3)^2=8/27
Two out of four times * two times * no two times
C(4,2) (2/3)^2 * (1-2/3)^2=8/27
Two out of four times * two times * no two times