79*69/80
=(80-1)*69/80
=80*69/80-1*69/80
=69-69/80
=68又11/80那9/20*101*9/20？9/20*101*9/20=9/20*(100+1)*9/20=81/400*100+81/400*1=8100/400+81/400=8181/400