11.2

Part of 1994 All-Russian Olympiad Regional Round

Problems(1)

at most 1 phone call All-Russian MO 1994 Regional (R4) 11.2

Source:

It was noted that during one day in a town, each person made at most one phone call. Prove that the people in the town can be divided into three groups such that no two persons in the same group talked by phone that day.