https://cis.temple.edu/~latecki/Courses/CIS2033-Spring12/ElementaryProbabilityforApplications/ch3.pdf
The first step in analyzing craps is to compute the probability that the player makes his point. Suppose his point is 5 and let Ek be the event that the sum is k. There are 4 outcomes in E5 ((1, 4), (2, 3), (3, 2), (4, 1)), 6 in E7, and hence 26 not in E5 ∪ E7. Letting × stand for “the sum is not 5 or 7,” we see that 4 26 4 = P(5) P(× 5