(20-2/55×1)+(19-2/55×2)+(18-2/55×3)+…+(1-2/55×20)
=(20+19+18+……+1)-2/55*(1+2+3+4+……20)
=(21×20÷2)×(1-2/55)
=210×53/55
=2226/11
(20-2/55×1)+(19-2/55×2)+(18-2/55×3)+…+(1-2/55×20)
=(20+19+18+……+1)-2/55*(1+2+3+4+……20)
=(21×20÷2)×(1-2/55)
=210×53/55
=2226/11