3x+0.5x=4.2
3.5x=4.2
x=4.2÷3.5
x=1.2
4x-2.16=1.2
4x=3.36
x=0.84
x-0.2x=16
0.8x=16
x=20
2x-4x乘以2.5=3.6
2x-10=3.6
2x=13.6
x=6.8
3x+5x=168
8x=168
x=21