12.5+6x=13.1
6x=13.1-12.5
6x=0.6
x=0.1
x-(3/20)x=18.7
(20/20)x-(3/20)x=18.7
(17/20)x=18.7
x=18.7÷17/20
x=18.7×20/17
x=22
2x+1.5x=17.5
3.5x=17.5
x=5
x-(1/2)x=4
(2/2)x-(1/2)x=4
(1/2)x=4
x=4÷1/2
x=8