7/12-9/20+11/30-13/42+15/56-17/72+19/20
=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8-1/8-1/9+19/20
=1/3-1/9+19/20
=2/9-19/20
=131/180
7/12-9/20+11/30-13/42+15/56-17/72+19/20
=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8-1/8-1/9+19/20
=1/3-1/9+19/20
=2/9-19/20
=131/180