0.25*7.9*0.5=1/4*1/2*7.9=1/8*（8-0.1）=1/8*（8-1/10）=1/8*8-1/8*1/10=1-1/80=79/80
0.25*7.9*0.5=0.25*(8-0.1)*0.5=0.25*(4-0.05)=1- 0.0125 =0.9875
