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Mark Smith works on the 13th floor of a 15 floor office building. There is only one elevator. The elevator moves continuously through floors one to 15 and then back down to one.

The elevator only stops on a floor on which the button has been pressed. It does not change direction unless it is at the top or the bottom of the building.

Assume that time spent loading and unloading passengers is very small compared to the travelling time.

Mark complains that at 5pm, when he wants to go home, the elevator is almost always going up when it stops on his floor. Is he correct? What is the explanation for this?


With thanks to the Tesco commercial science team.