分母的共积（专业名词早就忘记了）是60
原题可以转换成30/60+10/60+5/60+3/60+2/60=50/60=5/6
能简算：
1/2+1/6+1/12+1/20+1/30
=1-1/2+1/2+1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1-1/6
=5/6
=6分之5
1/2+1/6+1/12+1/20+1/30+1/42
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7
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