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 101

  1. Select a 2-digit number .
  2. Write it twice!

    Examples:

    1. 47 x 101 = 4747.
    2. 38 x 101 = 3838.
    3. 96 x 101 = 9696.

   Multiplying a 3-digit number by 101

  1. Select a 3-digit number .
  2. The sum of the first and third digits will be the middle digit: _ _ X _ _.
  3. The first two digits plus the carry will be the first digits: X X _ _ _.
  4. The last two digits of the number will be the last digits: _ _ _ X X.

   Example:

  1. The number chosen is 318.
  2. 3 + 8 = 11 (sum of first and third digits):
    _ _ 1 _ _ (keep carry, 1)
  3. 31 + 1 = 32 (first two digits plus carry):
    3 2 _ _ _.
  4. The last two digits are the same:
    _ _ _ 1 8.
  5. So 318 x 101 = 32118.

   See the pattern?

  1. If the number chosen is 728:
  2. 7 + 8 = 15 (sum of first and third digits):
    _ _ 5 _ _ (keep carry, 1)
  3. 72 + 1 = 73 (first two digits plus carry):
    7 3 _ _ _.
  4. The last two digits are the same:
    _ _ _ 2 8.
  5. So 728 x 101 = 73528.


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14 January 1996
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