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stolyar, what about (0,20) & (20,0)...?
Answer: 21
another way to look at it is y = -x + 20.
Equation of a line that cuts Y axis at 20. Now from origin(i.e. 0) to 20 are the number of possible positive solutions to this equation (i.e. 21).
thanks.

OK, but 0 is neither positive nor negative. You should have known this fact. The initial question clearly demands to find only positive solutions. So, I insist on my answer.