13.257. A courier on a moped left city $A$ for city $B$, which are 120 km apart. One hour after this, a second courier on a motorcycle left $A$ and, catching up with the first, immediately handed him an errand and then turned back, returning to $A$ at the moment the first courier reached $B$. What is the speed of the first courier if the speed of the second is 50 km/h?