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 1 1/2

  1. Select a 2-digit number.
  2. Multiply by 3.
  3. Divide by 2.

   Example:

  1. The 2-digit number chosen to multiply by 1 1/2 is 63.
  2. Multiply by 3: 3 x 63 = 180 + 9 = 189
  3. Divide by 2: 189/2 = 94 1/2
  4. So 63 x 1 1/2 = 94 1/2.

   See the pattern?

  1. If the 2-digit number chosen to multiply by 1 1/2 is 97:
  2. Multiply by 3: 3 x 97 = 270 + 21 = 291
  3. Divide by 2: 291/2 = 145 1/2
  4. So 97 x 1 1/2 = 145 1/2.

Remember, if you use a calculator you will get a decimal approximation, whereas the fractional form will be exact.


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16 August 1999
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