1/30+1/42+1/56+1/72+1/90
=1/5*6+1/6*7+1/7*8+1/8*9+1/9*10
=(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/5-1/10
=0.1