After a computer virus entered the system, a computer manager checks the condition of ill important files. She knows that each file has probability 0.2 to be damaged by the virus, nclependently of other files.

(a) Compute the probability that at least 5 of the first 20 files are damaged.

(b) Compute the probability that the manager has to check at least 6 files in order to find 3 undamaged files.