7.5+2.5X3.6
=7.5+2.5X（4×0.9）
=7.5+（2.5×4）×0.9
=7.5+10×0.9
=7.5+9
=16.5
7.5+2.5X3.6=7.5+2.5X3x1.2=7.5x(1+1.2)=7.5x2.4=18
如图
