5 coins, 2 of them fake (2016 Auckland MO J1)
Source:
November 13, 2021
weightcombinatorics
Problem Statement
It is known that in a set of five coins three are genuine (and have the same weight) while two coins are fakes, each of which has a different weight from a genuine coin. What is the smallest number of weighings on a scale with two cups that is needed to locate one genuine coin?