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