Multiplication by 5

Often, instead of multiplying by 5, it is more convenient to multiply first by 10 and then divide the result by 2. For example,

137·5 = 1370/2 = 685.

Another examples:

342·5 = 3420/2 = 1710.

But how do you divide by 2 fast? One simple way is to split the number into parts so that each is divided by 2 easily. In the above cases:

 1370/2= (1200 + 100 + 70)/2
  = 600 + 50 + 35
  = 685.

and also

 3420/2= (3000 + 400 + 20)/2
  = 1500 + 200 + 10
  = 1710.

Note how useful here was the distributive law.


Related material
Read more...

  • Multiplication by 9, 99, 999, (Multiply + Subtract) etc.
  • Squaring 2-Digit Numbers
  • Division by 5
  • Multiplication by 2
  • Multiplication by 9, 99, 999, etc. (Something Special)
  • Product of 10a + b and 10a + c where b + c = 10
  • Product of numbers close to 100
  • Product of two one-digit numbers greater than 5
  • Product of 2-digit numbers
  • Squaring Numbers in Range 26-50
  • Squaring Numbers in Range 51-100
  • Squares of Numbers That End in 5
  • Squares Can Be Computed Squentially
  • How to Compute Fast Any Square
  • Adding a Long List of Numbers
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