7/30
1/12+1/20+1/30+……+1/90=1/3 -1/4+1/4-1/5 +1/5-1/6+~~~~~+1/9-1/10=1/3-1/10=7/30
1/12+1/20+1/30+……+1/90=1/3-1/4+1/4-1/5+1/5-1/6+……+1/9-1/10=1/3-1/10=7/30
