1/42+1/56+1/72+1/90
=(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/6-1/10
=1/15