so # is just *
x = 5a
y = 5b
a+b=3
xy = 25ab
b=3-a
xy = 25a(3-a)
xy = 75a-25a^2
also, xy = 75b - 25b^2
(whichever way you want to word it)
there are 4 variables (a, b, x, y) but only 3 equations (x=5a, y=5b, a+b=3), so we don't get a regular number as an answer.