六分之五减十二分之七加二十分之九减三十分之十一加四十二分之十三减五十六分之十五加七十二分之十七是多

原题为
5/6-7/12+9/20-11/30+13/42-15/56+17/72
解=10/12-7/12+9/20-11/30+13/42-15/56+17/72
=3/12+9/20-11/30+13/42-15/56+17/72
=1/4+9/20-11/30+13/42-15/56+17/72
=5/20+9/20-11/30+13/42-15/56+17/72
=14/20-11/30+13/42-15/56+17/72
=7/10-11/30+13/42-15/56+17/72
=21/30-11/30+13/42-15/56+17/72
=10/30+13/42-15/56+17/72
=1/3+13/42-15/56+17/72
=14/42+13/42-15/56+17/72
=27/42-15/56+17/72
=9/14-15/56+17/72
=36/56-15/56+17/72
=21/56+17/72
=3/8+17/72
=27/72+17/72
=44/72
=11/18
因此,上式结果为十八分之十一.