Suppose users share a 3Mbps link

 Suppose users share a 3Mbps link. Also suppose each user requires 1Mbps when transmitting, but each user transmits only 25% of the time. (See the discussion of packet switching versus circuit switching in Section 1.3)
a, (1 points) When circuit switching is used, how many users can be supported?
b, (2 points) For the remainder of this problem, suppose packet switching is used. Find the probability that a giving user is transmitting.
c, (3 points) Suppose there are 5 users, find the probability that at any given time, all 5 users are transmitting simultaneously. What is the probability that there are 4 or less users transmitting simultaneously?