1/3+2/5+3/7+9/20+11/28+17/30+25/42
=(1/3+17/30)+(2/5+9/20)+(3/7+11/28)+25/42
=9/10+17/20+23/28+25/42
=5/4+23/28+25/42
=29/14+25/42
=8/3
=2又2/3