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.


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