multiple random variables

Select integrated circuits, test them in sequence until you find the first
failure, and then stop. Let N be the number of tests. All tests are independent with probability of failure p = 0.1. consider the condition B = N ≥ 20 .
(a) Find the PMF of PN(n)
(b) Find PN|B(n) the conditional PMF of N given that there have been 20
consecutive tests without a failure.
(c) What is E[N|B] , the expected number of tests given that there have been 20
consecutive tests without a failure.