4x-3(20-x)=5x-7(20-x)
4x-60+3x =5x-140+7x
7x-60=12x-140
7x-7x-60=12x-7x-140
-60=5x-140
-60+60=5x-140+60
0=5x-80
(+)80=5x
x=80/5
x=16