Problem for Discrete Math:

14 A student forgot the last digit of a phone number she was about to dial. She decided to dial the last digit at random. What is the probability that the student needs at most three trials? Hint. Let A ~ the student needs at most three trials. Then A\" ~ the student needs more than three trials and show that P (Ac) = P{she fails in trial 1 and she fails in trial 2 and she fails in trial 3} = P{F1 F1 F2 0.1713}: P{F1}P{F2|F1}P{F3|F1 F2}