Detailed explanation of probability theory and mathematical statistics edited by He Shuyuan

We must make separate requests. Answer: There are 20 units as a group, and the probability that there is no fault or one unit is faulty is m (omitted, you should ask the questioner), so the probability that it cannot be repaired in time is 1-m, so the total number is 5*( 1-m), which is the probability to be asked.

B is simpler, just calculate the failure probability of 3 sets, 2 sets, 1 set, 0 sets, n,1-n.