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# Hidden Treasures

### Calendar Capers

### Counting Factors

### Latin Numbers

### Building Tetrahedra

### Gift of Gems

### Rationals Between...

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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!

Age 11 to 14

Challenge Level

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

Age 11 to 14

Challenge Level

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

Age 14 to 16

Challenge Level

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

Age 14 to 16

Challenge Level

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

Age 14 to 16

Challenge Level

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

Age 14 to 16

Challenge Level

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