If the constraints on x,y are >or = to zero, the optimal solution is
F(z) =90
x = 0
y = 15
If the constraints must be x,y only > 0, and x & y are non-integer, then the optimal solution would be
x slightly greater than 0.... say .0001
Then y would be 14.9999 etc.
This would make f(z) = 89.9999 approximately