x-1/2x=42
1/2x=42
x=84
x-2/5x=3/8
3/5x=3/8
x=3/8÷3/5
x=5/8
x+0.8x=36
1.8x=36
x=20
45-1/2x=5
1/2x=45-5
1/2x=40
x=80
如果本题有什么不明白可以追问,