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

Mathematics

Kyleigh Meyers

13 December, 19:18

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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Tatiana Carter · 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