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Painter Pete, our local DIY expert, is renovating the local hospital.

He has to number each of the rooms in the hospital with plastic numbers.

However, he has run out of the number 9.

How many 9's must Pete buy in order to number all 1,000 rooms?

Puzzle Copyright © Kevin Stone

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Workings

Hint
Don't forget the extra 9's in 90, 91, etc.

Answer
300.

Reasoning
Start with: 9, 19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 – which is 20 9's.

We then repeat this for 100, 200, 300, up to 900 – which is 200 9's.

But from 900 to 999 we have an extra 100 9's as each starts with a 9.

So the total is 300.

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