BEATCALC:  Beat the Calculator!

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Back to Calculation Tips & Tricks
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From the BEATCALC mailing list:
a weekly service from B. Lee Clay, archived here by the Math Forum.
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   Multiplying a 2- or 3-digit number by 16 2/3

  1. Select a 2-digit number. (Choose larger numbers when you feel sure about the method.)
  2. Add two zeros to the number.
  3. Divide by 6 (or by 2 and 3).

   Example:

  1. The 2-digit number chosen to multiply by 16 2/3
    is 96.
  2. Add two zeros: 9600.
  3. Divide by 3: 9600/3 = 3200. (9600 is divisible by 3 because the sum of its digits is divisible by 3.)
  4. Divide by 2: 3200/2 = 1600.
  5. So 96 x 16 2/3 = 1600.

   See the pattern?

  1. If the 2-digit number chosen to multiply by 16 2/3
    is 76:
  2. Add two zeros: 7600.
  3. Divide by 2: 7600/2 = 3800.
  4. Divide by 3: 3800 / 3 = 1266 2/3.
  5. So 76 x 16 2/3 = 1266 2/3.

Test the number to see if it is divisible by 3. If not, divide by 2 first. With some practice you will become adept at multiplying by 16 2/3


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12 August 1996
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