5/6-1/12-9/20+11/30-1/42
=1/2+1/3-(1/3-1/4)-(1/4+1/5)+(1/5+1/6)-(1/6-1/7)
=1/2+1/3-1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7
=1/2-1/7
=(7-2)/14
=5/14