Question

A bicycle lock has a 4-digit combination. Each of the digits is a number from 0 to 9. Find the probability that the lock has a combination in which 3 of the 4 digits are 3s

Answer 1

I will assume that repeats are allowed as in most combination locks
so number of possible codes = 10^3 = 1000

So we want three 3's and one non-3
333x
33x3
3x33
x333---> where x is a number other than 3
so there are 4(9) of these or 36

so prob of our event = 36/1000 = 9/250

Answer 2

Reiny was close, I would adjust the possible combination to 10^4 = 10,000

So 36/10,000 = 9/2,500