Useful Number Facts For Quick Check Calculations

When I'm doing my quick check calculations, I sometimes need some facts which are not given with the numbers. I've collected some of the useful, but not necessarily obvious, number facts which I use for my calculations. There are lots more. If you know of others, write to the Mathemagician to let me know.
There is divinity in odd numbers, either in nativity, chance, or death.
- William Shakespeare, "The Merry Wives of Windsor"
- Some guidelines for 'quick check' calculations.
- Numbers in the real world.
- Grannus' circle.
- The front gate of Esmerel.

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