the probability of x=10 when r = 8.
f(10) =c(10 -1, 8 - 1) (0.8)8(0.2)2= 36(0.8)8(0.2)2, which is approximately 24%.
we could then ask what is the average number of free throws shot before this player makes eight of them. since the expected value is 8/0.8 = 10, this is the number of shots. 