Work Analysis and Design Question

Q2. Suppose at a call service center you have one phone operator to answer: n = 4 phone lines p=0.10 - probability that a phone line is in use is 9-1-p=0.9 = the probability that a given phone line is unused, e.g. no one on the line. "In use" means that a caller may be either: waiting on the line, or speaking with the phone operator. If calls come into the center at random, what is the probability that there will be exactly three phone lines in use? (e.g. One caller speaking with the operator, two callers waiting). Find only probabilities