Question 922143
The equation can represent a line, and the equation has infinitely many solutions, so x+y is not any specific number.  BUT you want INTEGERS x and y.


{{{(x/8+y/5)*40=31}}}
{{{5x+8y=31}}}
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What happens if y is solved in terms of x and x is solved in terms of y?
{{{8y=-5x+31}}}
{{{y=-(5/8)x+31/8}}}
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{{{5x=31-8y}}}
{{{x=31/5-8y/5}}}
{{{x=-(8/5)y+31/5}}}
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What is sum, x+y?
{{{-(8/5)y+31/5-(5/8)x+31/8}}}..........


Maybe making a graph, identifying some integer (x,y) points, and finding x+y for each of those would be easier, but would it give you better information?