A combination lock on a suitcase has 3 wheels, each labeled with nine digits from 1 to 9. If an opening combination is a particular sequence of three digits with no repeats, what is the probability of a person guessing the right combination?
As repetition is not allowed total no.of cases possible is
9
8
7(because if one of the numbers occupies a wheel, then the other wheel cannot be occupied by this number, i.e. next wheel have 1 less case than the previous wheel and so on)
Therefore, total cases = 504
Desired output is the correct combination of a single 3 digit number.
Therefore, the total no.of desired outcomes are 1
We know that,
Probability of occurrence of an event 
Therefore, the probability of correct combination 
Conclusion: Probability of guessing right combination is 