Question 93
There are four countries, each consists of several cities. The cities of any two countries are connected by at least $\frac{5}{6}$ of a number of all possible roads between these two countries. Prove that it is possible to choose one city from each country so that any two of them are connected. Any pair of cities can be connected by no more than one road.
(Nazar Serdyuk)
(Nazar Serdyuk)