# 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 are about 15 thousand possible three-letter combinations.
• The volume of a sphere is half the volume of the same size cube
• There are a million seconds in two weeks (1,209,600)
• There are 100,000 seconds in a day (actually 86,400)
• There are 1,000 hours in a month (720 or 744)
• There are 10,000 hours in a year (actually 8,760)
• There are 250 working days in a year.
• There are about 2000 working hours in a year.
• There are about 6 billion people in the world (or 5 billion, for the purpose of easier calculation)
• People live to be about 100 years old (for easy calculation)
• The Earth is about 24 or 25 thousand miles around
• The radius of the Earth is about 6000 km
• The surface area of the Earth is about 400 trillion square metres
• The mass of the Earth is about 6 x 10^24 kg
• The mass of the sun is about one million earths
• It costs about \$1000 a month to rent a billboard.

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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