text: ## BEATCALC:  Beat the Calculator!
  
  Back to Calculation Tips & Tricks
  
   From B. Lee Clay
  
  Squaring Numbers
  
  Multiplying Numbers
  
  Dividing Numbers
  
  Subtracting Numbers
  
  Finding Percents
  
  Calculation
  Practice Exercises
  
  Full List
  
  ### Multiplying a 2-digit number by 79 1/2
  
  1. Select a 2-digit number.
  2. Divide the number by 2.
  3. Subtract this from 80 times original number.
  
  #### Example:
  
  1. The 2-digit number chosen to multiply by 79 1/2 is 21.
  2. Divide by 2: 21/2 = 10 1/2.
  3. Subtract this from 80 times original number: 80(21) - 10 1/2 = 1600 + 80 -10 1/2 = 1680 - 10 1/2 = 1670 - 1/2 = 1669 1/2.
  4. So 79 1/2 × 21 is 1669 1/2.
  
  See the pattern?
  
  1. The 2-digit number chosen to multiply by 79 1/2 is 66.
  2. Divide by 2: 66/2 = 33.
  3. Subtract this from 80 times original number: 80(66) -33 = 4800 + 480 - 33 = 4800 + 450 - 3 = 5250 - 3 = 5247.
  4. So 79 1/2 × 66 is 5247.
  Multiply left to right in easy to handle increments and you will be able to give these products without a calculator.