# 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 Some guidelines for 'quick check' calculations. Numbers in the real world. Grannus' circle. The front gate of Esmerel.