Question

A maintenance man has 12 keys on his key ring. if he tries the keys at random on a storage room door, discarding those that do not work, what is the probability that he will get the door open on his 4th try?

Answer 1

P(Failure on first try) = 11/12
P(Failure on second try) = 10/11
P(Failure on third try)= 9/10
P(success on fourth try) = 1/9

P(success exactly on fourth try) = 11/12*10/11*9/10*1/9 = 1/12