I have a lockbox with the digits 0-9 on it. The passcode is 5 digits, repeats are not allowed, and order doesn't matter (I just have to have the correct 5 digit buttons depressed).
A. If I guess at the code, what is the probability that the box will open?
B. If I have completely forgotten the code, what is the probability that it will take less than 10 tries to get it open?
There are 10 digits and digits cannnot be repeated and order does not importanat so number of possible different passcodes is:
C(10,5) = 10! / (5! *5!) = 252
A:
Out of these possible passcodes only one is correct so the probability that the box will open is a try is
P(open) = 1 / 252= 0.003968
B:
Out of these 252 possible passcodes 251 are correct and 1 is correct. The probability that it will take less than 10 tries (that is 1, 2, 3..or 9 tries) to get it open is
Hence, the required probability is 0.0357143.
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