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?

Question

Butterball

Mathematics

21 July, 04:02

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?

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Skinner · 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