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 7 1/2

  1. Multiply by 3.
  2. Divide by 4.
  3. Add a zero or move the decimal point one place to the right.

   Example:

  1. If the 2-digit number chosen to multiply by 7 1/2 is 64:
  2. Multiply by 3: 3 x 64 = 180 + 12 = 192
  3. Divide by 4: 192/4 = 48
  4. Add zero: 480
  5. So 64 x 7 1/2 = 480.

   See the pattern?

  1. If the number to be multiplied by 7 1/2 is 93:
  2. Multiply by 3: 3 x 93 = 279.
  3. Divide by 4: 279/4 = 69 3/4 or 69.75
  4. Move the decimal point one place to the right: 697.5
  5. So 93 x 7 1/2 = 697.5


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6 April 1998
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