7/12-9/20+11/30-13/42+15/56
=(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8)
=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8
=1/3+1/8
=11/24
7/12-9/20+11/30-13/42+15/56
=(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8)
=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8
=1/3+1/8
=11/24