4.5×0.8-1.6×0.5=3.6-0.8=2.8
4.5*0.8-1.6*0.5=4.5*0.8-0.8*（2*0.5）=4.5*0.8-0.8*1=0.8*（4.5-1）=0.8*3.5=2.8
4.5×0.6-1.9×0.5。
