The rule to determine if a number is divisible by nine without using any division. Simply add up all the digits of the number. Repeat this until you come out with a one digit number. If this number is nine, the original number is divisible by nine.

Example:

9 * 98765 (yeah, I'm lazy) = 888885
8 + 8 + 8 + 8 + 8 + 5 = 45
4 + 5 = 9

8 * 36547 = 292376
2 + 9 + 2 + 3 +7 +6 = 29
2 + 9 = 11
1 + 1 = 2

I learned this back in the day from watching Square One on PBS.

On the show it had a little ditty that went along with it:

Nine, nine, nine,
Fantastic number nine.
Perfectly efficient,
It works out every time.
Nine, nine . . .

I can't believe I still remember that.

This trick is similar to how to find out if ANY number is divisible by eleven.

The Rule of Nines also is used in first aid to calculate the extent of burns.

Most parts of a human body consists of nine percent of the outer surface area.

  • Head: 9% (4.5% front, 4.5% back)
  • Chest to the lower rib cage: 9%
  • Abdomen, from rib cage to groin area: 9%
  • Back, from neck to bottom of rib cage: 9%
  • Lower back, from bottom of rib cage to below buttocks: 9%
  • Front of left leg: 9%
  • Front of right leg: 9%
  • Back of left leg: 9%
  • Back of right leg: 9%
  • Left arm: 9% (front: 4.5%, back: 4.5%)
  • Right arm: 9% (front: 4.5%, back: 4.5%)
  • Groin: 1%

When assessing the victim, count up each area with a burn. Figure out the percentage, then you can go to HOWTO: Treat thermal burns in the backcountry to figure out what to do.

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