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The answer is 16 C 2 =120.
The solution is (n-1) C (r-1) if 0 is not allowed and (n+r-1) C (r-1) if 0 is allowed.
@spideyunlimited
3^17 was a bad mistake. Whether u first put the 1 in x and then in y or
first in y and then in x , its the same. So you have counted the same cases again and again.
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