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Word Problem

  1. Oct 11, 2006 #1
    Ive got an assignment of word problems and cant seem to figure this one out:

    A 6 Litre car radiator is 2 thirds full of water. How much of a 90% antifreeze solution (90% is alcohol by volume) must be added to it to make a 10% antifreeze solution in the radiator?

    I came up with the equation: 0.9x=(0.1)(6)

    but that didnt work so any help about what the way to set up the problem would be great.
     
  2. jcsd
  3. Oct 11, 2006 #2

    Office_Shredder

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    First, this should probably be in the homework section, but oh well

    Second, you hvae to ask yourself a different question (which you don't seem to have taken into account)

    How much water is in the radiator?
     
  4. Oct 11, 2006 #3
    4L of water
     
  5. Oct 11, 2006 #4

    Office_Shredder

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    Ok, this whole posting in two places isn't going to cut it, so I'm just going to stick with this thread.

    If the anti-freeze solution is 90% antifreeze, then when you add a volume V of solution to a volume v of liquid, your total volume becomes V + v, and your anti-freeze portion is .9V. So how can this be applied to the problem?
     
  6. Oct 11, 2006 #5
    ah

    so the equation should be 4+.9V=6(.1)?
     
  7. Oct 11, 2006 #6
    is that correct?
     
  8. Oct 11, 2006 #7
    ok i tried that equation and got an answer of -3.77777 so that cant be the correct answer, so i am kind of stumped as to what to try next lol.
     
  9. Oct 11, 2006 #8

    Office_Shredder

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    You have to explain WHY you think that's the equation. I don't know why you think there should be a 6 at all, for example, so it's difficult to help you
     
  10. Oct 13, 2006 #9

    HallsofIvy

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    You seem to be thinking that the radiator must be filled. That's obviously incorrect- in order to fill the radiator, you would have to add 2 litres of 90% antifreeze which would NOT give you a 10% solution.

    Let x be the number of litres of antifreeze added. Then you have 4+ x litres of liquid in the radiator. Since the antifreeze solution is 90% anti-freeze, you have added 0.9x litres of antifreeze. For that to be 10% of the entire amount, you must have 0.9x= 0.1(4+ x). Solve that equation for x.
     
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