1/5*10+1/10*15+1/15*20+1/20*25+1/25*30+1/30*35
=1/5*(1/5-1/10+1/10-1/15+1/15-1/20+1/20-1/25+1/25-1/30+1/30-1/35)
=1/5*(1/5-1/35)
=1/5*6/35
=6/175