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-digit number by 15

  1. Select a 2-digit number.
  2. Add a zero to it.
  3. Divide it by 2.
  4. Add the number obtained by dividing by 2 to the last number.

   Example:

  1. The 2-digit number chosen to multiply by 15 is 62.
  2. Add a zero: 620.
  3. Find half (divide by 2): 620/2 = 310.
  4. add: 620 + 310 = 930.
  5. So 62 x 15 = 930.

   See the pattern?

  1. If the number to be multiplied by 15 is 36:
  2. Add a zero: 360.
  3. Find half (divide by 2): 360/2 = 180.
  4. add: 360 + 180 = 540.
  5. So 36 x 15 = 540.


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10 February 1997
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