2分之1×5分之1+5分之1×8分之1+.+17分之1×20分之1
=1/3×[(1/2-1/5)+(1/5-1/8)+……+(1/17-1/20)]
=1/3×(1/2-1/5+1/5-1/8+……+1/17-1/20)
=1/3*(1/2-1/20)
=1/3*9/20
=3/20