An IRS auditor randomly selects 3 tax returns from 55 returns of which six contain errors. What is the probability that she selects none of those containing errors?

Answer 1

Initially, 49 out of 55 returns do not contain errors.
So probability of selecting the first (first trial) correct return is 49/55.
After this, there are 48 correct returns out of 54, so....48/54
For all three trials to succeed, we multiply the probabilities of all three trials to get the required probability.