It's a difficult one to get your head around. Imagine a vehicle with a caterpillar track: where the track is in contact with the ground it is stationary and the vehicle moves along it, but the top of the track is moving (at twice the speed of the vehicle - giving an average speed of the track equal to that of the vehicle). Now imagine the track getting shorter: the same principal applies even when the track is so short that it is circular, like a wheel. Still doesn't help with your problem though.
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