0.8X-（X-7）=3
0.8X-X+7=3
0.8X-X=3-7
0.2X=4
X=4/0.2
X=200.2X怎么得来的0.8X-（X-7）=30.8X-X+7=30.8X-X=3-7-0.2X=-40.2X=4X=4/0.2X=20