0.75*15+4.5*2.5
=45/4+45/4=45/2=22.5
3.6*31.4+43.9*6.4
=（1-6.4）*31.4+43.9*6.4
=31.4-6.4*（43.9-31.4）
=31.4-32/5*5/2
=31.4-16
=15.4