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)=7
s=50/3
b.假设丙在甲的下游,则有:s/(7.5+2.5)+(10-s)/(7.5-2-5)=7
s无解
综上所述:s=50/3