5 coins, 2 fake - All-Russian MO 2000 Regional (R4) 10.2
Source:
September 26, 2024
combinatoricsweighings
Problem Statement
Among five outwardly identical coins, are real and two are fake, identical in weight, but it is unknown whether they are heavier or lighter than the real ones. How to find at least one real coin in the least number of weighings?