1/3+1/6+1/10+1/15+1/21+1/21+1/28
=1/3+(1/2-1/3)+(1/6-1/15)+1/15+(1/12-1/28)+1/21+1/28
=1/2+1/6+1/12+1/21
=3/4+1/21
=67/84