2/3-7/12+9/20-11/30+13/42-15/56
=2/3-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)
=2/3-1/3-1/8
=1/3-1/8
=5/24