21×17/20
=(20+1)×17/20
=20×17/20+1×17/20
=17+17/20
=17又17/20
19/54×53
= 19/54×(54-1)
=19/54×54-19/54×1
=19-19/54
=18+(1-19/54)
=18又35/54
21×17/20（简便算法） 19/54×53（简便运算）
21×17/20
=(20+1)×17/20
=20×17/20+1×17/20
=17+17/20
=17又17/20
19/54×53
= 19/54×(54-1)
=19/54×54-19/54×1
=19-19/54
=18+(1-19/54)
=18又35/54