To celebrate NRICH's 20th birthday, we've taken a look back in the archives and dusted off some problems from the early days, freshened them up, and added some new ideas. We hope you enjoy working on these hidden treasures!

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Choose any three by three square of dates on a calendar page...

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Is there an efficient way to work out how many factors a large number has?

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Can you create a Latin Square from multiples of a six digit number?

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Can you make a tetrahedron whose faces all have the same perimeter?

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Four jewellers share their stock. Can you work out the relative values of their gems?

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What fractions can you find between the square roots of 65 and 67?