7/12-9/20+11/30-13/42
=(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)
=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7
=1/3-1/7
=4/21