Using 2 out of 3 means of transportation
Source: Baltic Way 1993
June 15, 2012
combinatorics unsolvedcombinatorics
Problem Statement
There are cities in a certain kingdom. Between some pairs of the cities a two-way direct bus, train or plane connections are established. What is the least possible number of connections to be established so that choosing any two means of transportation one can go from any city to any other without using the third kind of vehicle?