①0.3x+6.5×3=31.2
0.3x+19.5=31.2
0.3x+19.5-19.5=31.2-19.5
0.3x÷0.3=11.7÷0.3
x=39
②0.5x+0.6x=19.91
1.1x÷1.1=19.91÷1.1
x=18.1
③0.5×40-2.5x=3.75
20-2.5x=3.75
20-2.5x+2.5x=3.75+2.5x
3.75+2.5x-3.75=20-3.75
2.5x÷2.5=16.25÷2.5
x=6.5
④0.3x+1.2×3=10.5
0.3x+3.6=10.5
0.3x+3.6-3.6=10.5-3.6
0.3x÷0.3=6.9÷0.3
x=23