Andrea’s only timepiece is a clock that’s fixed to the wall. One day she forgets to wind it and it stops.
She travels across town to have dinner with a friend whose own clock is always correct. When she returns home, she makes a simple calculation and sets her own clock accurately.
How does she manage this without knowing the travel time between her house and her friend’s?
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Before she leaves the house, Andrea winds her own clock and sets it to an arbitrary time. Then she notes the correct time at her friend’s house both when she arrives and when she leaves. When she returns home she consults her own clock to see how much time the whole trip has taken, subtracts the period she spent at her friend’s house, and divides the result by two to learn the travel time in each direction. By adding this interval to the time she noted as she left her friend’s house, she can infer the current time and set her own clock.
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