1/10+1/15+1/21+...+1/120
=2*(1/20+1/30+1/42+...+1/240)
=2*(1/4*5+1/5*6+...+1/15*16)
=2*(1/4-1/5+1/5-1/6+...+1/15-1/16)
=2*[(1/4-1/16)+(1/5-1.5)+...+(1/15-1/15)]
=2[(4/16-1/16)+0+...+0]]
=2*3/16=3/8
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