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'Coin Cogs' printed from http://nrich.maths.org/

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Here is a picture of a UK £2 coin with two pink circles drawn on it:
 

two pound coin

If you look at the band between the two pink circles, you will see that it contains lots of cogwheels.

Can you explain what is wrong with these cogs?

To get an idea of how cogwheels move, you could look at the interactivity in this month's Turning Cogs problem.