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 625

  1. Select a 2-digit number.
  2. Add a zero and divide by 2.
  3. Divide by 8.
  4. Move the decimal point 3 places to the right.

   Example:

  1. The 2-digit number chosen to multiply by 625 is 38.
  2. Add zero: 380; divide by 2: 380/2 = 190
  3. Divide by 8: 190/8 = 23 6/8 = 23.75
  4. Move the decimal point 3 places to the right: 23750
  5. So 625 x 38 = 23,750.

   See the pattern?

  1. The 2-digit number chosen to multiply by 693 is 72.
  2. Add zero: 720; divide by 2: 720/2 = 360
  3. Divide by 8: 360/8 = 45
  4. Move the decimal point 3 places to the right: 45000
  5. So 625 x 72 = 45,000.


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11 May 1998
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