rule of nines

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.

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.