Answer
The probability of guessing a lock combination on the first try is $\frac{1}{125,000}=0.000008$.
Work Step by Step
The sample space: 125,000 diffent combinations. So, N(S) = 125,000.
There is only one correct combination. So, N(correct combination) = 1.
Using the Classical Method (see page 259):
$P(correct~combination)=\frac{1}{125,000}=0.000008$
