X+X*20%-(1200-X)-(1200-X)*20%=40% ,等式左右同时乘以100得
100X+X*20-100*(1200-X)-(1200-X)*20=40
120X-(1200-X)*120=40
12X-(1200-X)*12=4
3X-(1200-X)*3=1
3X-3600+X*3=1
6*X=3601
X=3601/6 ,约等于600