How to find number of combinations, with restrictions? [closed]

Hello all,

I'm having trouble with a computer algorithm. In my class I've learned how to solve something like this

The worst hockey team can choose a single player from 3 teams. each team has 20, 21, 24 players respectively. How many players are there to choose from

And something like this

How many license plates can be made which have 3 numbers and then 3 letters.

The problem I am having is with a question like this (which i found in my textbook, for extra work):

How many strings of 4 digit decimals (1.0000) are there which don't contain the same digit 3 times?

I'm not sure how to solve with the condition. Can someone offer help?

Thanks!

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This is not appropriate for this site. Try math.stackexchange.com – alext87 Sep 16 2010 at 17:44
Wrong place to ask this question. Hint: how many strings of 4-digit decimals are there? (call that A) How many strings of 4-digit decimals contain the same digit 3 times? Call the answer to that B. The answer you seek is A minus B. – sleepless in beantown Sep 16 2010 at 18:18