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2000 balls, weigh 10 or 9.9 - All-Russian MO 2000 Regional (R4) 9.6

Source:

September 26, 2024
combinatoricsweighings

Problem Statement

Among 20002000 outwardly indistinguishable balls, wines - aluminum weighing 100 g, and the rest - duralumin weighing 9.99.9 g. It is required to select two piles of balls so that the masses of the piles are different, and the number of balls in them - the same. What is the smallest number of weighings on a cup scale without weights that can be done?