0.75*3.16+7.84*75%—75%
=0.75*3.16+7.84*0.75-0.75
=0.75*（3.16+7.84-1）
=0.75*10
=7.5
3x—120%x=0.45
3x-1.2x=0.45
（3-1.2）x=0.45
1.8x=0.45
x=0.25