Walking up a Hill
Jack begins to walk up a hill at 8 a.m. and reaches the top at noon. The next day, Jack backtracks along the same path, begins to walk down the hill at 8 a.m. and reaches the bottom at noon.
True or False?
There must have been a time between 8 a.m. and noon during those two days at which Jack was at the same position on the hill.
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Imagine one Jack walking up the hill and another down. Him walking up must meet him walking down at some point, which marks where and when he'd be at the same point.