Question 1177880: How many 4 digit numbers can be formed from the digits 0-9
Answer by MathLover1(20850)   (Show Source):
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if repetitions are allowed,
Two ways to get the answer:
1. There are integers starting with  and ending with  .
But the  integers starting with and ending with have less than  digits,
so the desired number is  or  ways.
2.
there is total of  digits from  to 
so, there is ways to choose each ones, tens, hundredths, and thousands'
that is 
since  cannot be in place of thousands', only ones, tens, hundredths which is  three digit numbers , deduct it
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