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**lovesmath** Another way to count the number of nonnegative integral solutions to an equation of the form x1+x2+...+xn=m is to reduce the problem to one of finding the number of n-tuples (y1,y2,...,yn) with 0<=y1<=y2<=...<=yn<=m. The reduction results from letting yi=x1+x2+...+xi for each i=1,2,...,n. Use this approach to derive a general formula for the number of nonnegative integral solutions to x1+x2+...+xn=m.

I'm not even sure where to start with this problem. I would appreciate any help.