I am very lost on this question. Any help is much appreciated. thank you!

Suppose users share a 3 Mbps link. Each user requires 150 kbps when transmitting, but each user transmits only 10% of the time.

(a) When circuit switching is used, how many users can be supported? (b) Assume packet switching is used for the rest of this problem. Find the probability that a given user is transmitting. (c) Suppose there are 120 users. Find the probability that exactly n users are transmitting simultaneously at any given time. (Hint: Use the binomial distribution.) (d) Find the probability that there are 21 or more users transmitting simultaneously.