1) x:3-18=12
1\3 x -18=12
x-54=36
x=36+54
x=90
2) 5*x+80=690
5x=690-80
5x=610
x=610:5
x=122
3) x*8+120=280
8x+120=280
8x=280-120
8x=160
x=160:8
x=20
