7.5*1.2=7.5*2*0.6=15*0.6=9
3.8*0.99=3.8*（1-0.01）=3.8-3.8*0.01=3.8-0.038=3.762
0.25*1.25*3.2=0.25*1.25*(4*0.8)=(0.25*4)*(1.25*0.8)=1
备注：*代表乘号