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