设甲乙两地的路程为s.a.假设丙在甲的上游，则有：s/(7.5+2.5)+(10+s)/(7.5-2.5)=7s=50/3b.假设丙在甲的下游，则有：s/(7.5+2.5)+(10-s)/(7.5-2-5)=7s无解综上所述：s=50/3
