SOLUTION: what are all of the possible combinations that can be made of 4 digits using numbers 0-9?

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Question 370261: what are all of the possible combinations that can be made of 4 digits using numbers 0-9?
Found 2 solutions by edjones, Year 11 kid:
Answer by edjones(8007) About Me  (Show Source):
You can put this solution on YOUR website!
nCr=combination of n things taken r at a time= n!/((n-r)!r!)
Arrangement is not important.
=10!/(6!*4!)
=210
.
Ed

Answer by Year 11 kid(1) About Me  (Show Source):
You can put this solution on YOUR website!
The answer of the another tutor is totally wrong.
As we know, "0" cant be the first digit, so there is 9 possible choice of the first place,as the question doesnt say we cant repeatly use the number, that we have 10 choices in the second, third and the last place,
there are 9x10x10x10=9000 ways of managing the digit.
P.s I am only a year 11 kid and I am an EAL students. Plz ignore the typos or grammar mistakes.