简便：(40.2*8.1*4/5)/(0.8*81*20.1)括号外的是除号！

(40.2*8.1*4/5)/(0.8*81*20.1)
=(2*4/5*81*0.1*20.1)/(0.8*81*20.1)
=2*0.1
=0.2

4/5=0.8,和分母中0.8约掉
40.2/20.1=2
8.1/81=0.1
所以：(40.2*8.1*4/5)/(0.8*81*20.1)=0.2

简便：(40.2*8.1*4/5)/(0.8*81*20.1)
=(2×20.1×8.1×0.8）/（0.8×8.1×10×20.1）
=2/10
=0.2