1/42+1/56+1/72+1/90
=1/6*1/7+1/7*1/8+1/8*1/9+1/9*1/10
=1/7*(1/6+1/8)+1/9*(1/8+1/10)
=1/7*7/24+1/9*9/40
=1/24+1/40
=1/8*1/3+1/8*1/5
=1/8*8/15
=1/15