1/8+1/24+1/48+1/80+1/120
=1/2X4+1/4X6+1/6X8+1/8X10+1/10X12
=1/2(1/2-1/4 +1/4+1/6 +...+1/10-1/12)
=1/2(1/2-1/12)
=5/24