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Quickly Dividing in Two


Date: 7/8/96 at 8:39:27
From: Anonymous
Subject: Quickly Dividing in Two

I work in a store where everything is half price.  Are there any quick 
tricks for figuring out fifty percent of a number such as 14.95 in 
ones head?


Date: 7/8/96 at 12:33:44
From: Doctor Jodi
Subject: Re: Quickly Dividing in Two

Hi there! I'm not sure if this is a "quick trick" or not, but here are 
several ideas.  Some are variations on the same theme:

- round UP to the nearest whole number, which is 15
- divide by two (to get 15/2 or 7.50)
- now subtract 15 - 14.95, to get .05
- divide .05 by two (to get .025) and subtract this from 7.50 to 
  get 7.475 (the store will probably round it to 7.48)


Another idea:

- round DOWN to the nearest whole number, which is 14
- divide by two (to get 7)
- now subtract 14.95 - 14 to get .95
- divide .95 by two to get .475 and add this to 7 to get 7.475 
  (the store will probably call this 7.48)

Those methods recalled that 50 percent is also 1/2.  You can also use 
the fact that 50 percent is the same as 10 percent times 5:

Since 10 percent is the same as 10/100 or 1/10, first divide by ten 
(which amounts to moving the decimal point):

- divide by ten to get 14.95/10, which is 1.495
- now multiply by five to get 7.475 and round to 7.48

You can also combine variations on the methods above (like multiplying 
1.5 by 5 and then subtracting .005 times 5).

I'm not sure these are the best methods, but I use them regularly.  I 
hope they help.  Let us know if you think of any other ways to do this 
in your head.

-Doctor Jodi,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/   
    
Associated Topics:
Elementary Division
Middle School Division

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