If x and y are positive integers for which \(3x + 2y + xy = 115 + 7x + 6y\), then what is x + y?
Rearrange as
4x + 4y - xy + 115 = 0
4y - xy = -4x - 115 multiply through by -1
xy - 4y = 4x + 115
y [ x - 4] = [ 115 + 4x ]
y = [ 115 + 4x ] / [ x - 4]
Note that when x = 5, y = 135
x + y = 5 + 135 = 140
Thanks CPhill!
+86 
