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There and Back
Age 14 to 16
Challenge Level 
Brian swims at twice the speed that the river is flowing. He swims directly downstream, from one moored boat to another and then back again, taking 12 minutes altogether. How long would it have taken him in still water?
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Escalator
At Holborn underground station there is a very long escalator. Two people are in a hurry and so climb the escalator as it is moving upwards, thus adding their speed to that of the moving steps. ... How many steps are there on the escalator?
Illusion
A security camera, taking pictures each half a second, films a cyclist going by. In the film, the cyclist appears to go forward while the wheels appear to go backwards. Why?