x+40%=20
x=20-40%
x=20-0.4
x=19.6
2.x=8+4.5-8?
应该是x=8+4.5-5
x=7.5
3. 7.8=3x+3.6
-3x=3.6-7.8
-3x=-4.2
x=1.4