# probality of combination lock HELP!!

• Nov 13th 2012, 07:22 PM
ossie4
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
• Nov 13th 2012, 07:30 PM
MarkFL
Re: probality of combination lock HELP!!
There is only 1 right combination, but $10\cdot9\cdot8=720$ possible combinations, so the probability in question is:

$P(X)=\frac{1}{720}=0.0013\bar{8}\approx0.0014$
• Nov 13th 2012, 07:37 PM
ossie4
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
• Nov 13th 2012, 07:54 PM
MarkFL
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. :)