Question 18
The river city of Platense consists of several platforms and bridges between them. Each bridge connects two platforms and no two bridges are connecting the same two platforms. The mayor wants to change some bridges through a series of moves as follows: if there are three platforms $A$, $B$ and $C$, and bridges $AB$ and $AC$ but not $BC$, then $AB$ can be changed to $BC$.
A bridge configuration is *good* if you can go from any platform to any other using only the bridges. Starting from a good configuration, show that the mayor can reach any other good configuration, whose number of bridges is the same, through the movements described.
A bridge configuration is *good* if you can go from any platform to any other using only the bridges. Starting from a good configuration, show that the mayor can reach any other good configuration, whose number of bridges is the same, through the movements described.