# Thread: probality of combination lock HELP!!

1. ## probality of combination lock HELP!!

A small combination lock has 3 wheels, each labeled with the 10 digits from 0-9. If an opening combination is a particular sequence of 3 digits with no repeats, what is the probability of a person guessing the right combination.

The answer in the back of my book says 1/P 10,3 = .0014

How did they get that I keep getting 720 when I permutaation 10P3.. what do I do

2. ## Re: probality of combination lock HELP!!

There is only 1 right combination, but $\displaystyle 10\cdot9\cdot8=720$ possible combinations, so the probability in question is:

$\displaystyle P(X)=\frac{1}{720}=0.0013\bar{8}\approx0.0014$

3. ## Re: probality of combination lock HELP!!

Thank you so much I didn't round that was my issue.. been at it for a while and you cleared it up good post

4. ## Re: probality of combination lock HELP!!

Yes, you did it correctly, so I wanted to show another approach, and at the same time imply that rounding was the only issue.