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3 palaces, each rotating on a duck leg

Source: (2021-) 2022 XV Dürer Math Competition Finals Day2 E3

December 1, 2022
algebranumber theory

Problem Statement

Three palaces, each rotating on a duck leg, make a full round in 3030, 5050, and 7070 days, respectively. Today, at noon, all three palaces face northwards. In how many days will they all face southwards?