六种,20的因数有1,2,4,5,10,20
∵1+1=2,∴1/20=1/40*(1+1)=1/40+1/40
∵1+1/2=3/2,∴1/20=1/30*3/2=1/30*(1+1/2)=1/30+1/60
∵1+1/4=5/4,∴1/20=1/25*5/4=1/25*(1+1/4)=1/25+1/100
∵1+1/5=6/5,∴1/20=1/24*6/5=1/24*(1+1/5)=1/24+1/120
∵1+1/10=11/10,∴1/20=1/22*11/10=1/22*(1+1/10)=1/22+1/220
∵1+1/20=21/20,∴1/20=1/21*21/20=1/21*(1+1/20)=1/21+1/420