①5(x-5)=37.5
x-5=7.5
x=7.5+5
x=12.5
②7.5x+30=2.5x+100
7.5x-2.5x=100-30
5x=70
x=14
③11(x-3)/0.4=5.5
(x-3)/0.4=0.5
x-3=0.2
x=3+0.2
x=3.2