Can someone explain this solution? Why there is -4 and 6 ?

P8. Suppose users share a 3 Mbps link. Also suppose each user requires 150 kbps when transmitting, but each user transmits only 10 percent of the time. (See the discussion of packet switching versus circuit switching in Section 1.3.) a. When circuit switching is used, how many users can be supported? b. For the remainder of this problem, suppose packet switching is used. Find the probability that a given user is transmitting. exactly n users are transmitting simultaneously. (Hint: Use the binomial distribution.) dFind the probability that there are 21 or more users transmitting simultaneously