x = residential element
y = commercial element
The maximum of the commercial element (y) is 40% of the sum of x and y, i.e.:
y <= 0.4(x + y)
So, if y > 0.4(x + y), we can use the formula to maximise y:
y = 0.4(x + y)
Rearranging:
y = 0.4x + 0.4y
0.6y = 0.4x
y = (0.4/0.6) x
y = 2/3 x...