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Percentage help

  1. Apr 13, 2009 #1

    If I am calculating the percentage of a number lets say 10% of 20000 which is 18000
    Then I take 20% of 18000 which is 14400

    Why is this not the same as taking 30% of 20000? When I take 30% of 20000 I get 14000. Why is there is a difference of 400?

  2. jcsd
  3. Apr 13, 2009 #2


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    In order to take away 10% of 20000, we multiply: 0.90*20000=18000.

    to take 20% away from 18000, we have: 0.80*18000=0.80*(0.90*20000)=0.72*20000=14400.

    Thus, to take away 10% of 20000, and then take away 20% FROM THE REMAINDER, is the same as taking 28% of the ORIGINAL AMOUNT (i.e, 20000)!

    This is NOT the same as taking away 30% of the original amount!
  4. Apr 14, 2009 #3
    [tex]0.90 \cdot 0.80 \cdot 20,000 \ne 0.70 \cdot 20 000[/tex]

    [tex]0.90 \cdot 0.80 = 0.72 \ne 0.70[/tex]
  5. Apr 14, 2009 #4


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    10% of 20000 means one tenth of 20000. Partly using number characters to express that, you ask for [tex]\frac{1}{10}[/tex] of 20000.
    That in effect is expressed as [tex]\frac{1}{10}[/tex][tex]\times[/tex]20000

    EDIT: I am looking for a way to edit that after it's already published; I know it looks bad. There is no tools button field and I do not have the typeset tags memorized. I want a space between the fraction and the multiplication symbol and I want a space between the multiplication symbol and the 20000.
    Last edited: Apr 14, 2009
  6. Apr 14, 2009 #5
    You could use \cdot like I did :)
    BTW, why don't you just make one Latex code ?
  7. Apr 14, 2009 #6


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    My description broke the work into two separate steps; the second one looks bad. The description during the bulk of my post was intended to be descriptive and helpful.

    I see what you mean, to put the tex code all in one piece. If I still had my texAid program in use this would be very easy.
  8. Apr 14, 2009 #7

    Here are the commands for extra space in LaTeX, in order from smallest space to largest.

    Code (Text):

  9. Apr 15, 2009 #8
    Another way to think about it...

    Suppose you go to a store and see an item on sale for 50% off. Then you go to the store the next day and see that it's been reduced 50% more. Even so, the item isn't free!

    The store isn't adding the fifty percents. It's multiplying them.
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