3x+2(100-x)=300
3x+200-2x=300
x=100
4x-3(20-x)=6x-7(11-x)
4x-60+3x=6x-77+7x
7x-60=13x-77
6x=17
x=17/6
x+18/90=x-180/60
两边乘180
2x+36=3x-540
x=576
1-(2y-5)/6=3-y/4
两边乘12
12-4y+10=36-3y
y=-14