六分之五减十二分之七加二十分之九减三十分之一加四十二分之十三
=5/6-7/12+9/20-1/30+13/42
=1/4+9/20-1/30+13/42
=7/10-1/30+13/42
=2/3+13/42
=28/42+13/42
=41/42