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Homework Help: Algebra and %

  1. Sep 6, 2004 #1
    How much water must be added to a gallon of alcohol 75% pure to make a mixture 15% pure ?

    The book I'm reading has given this formula : 0.75/(n+1)= 0.15 while n is the number of gallons to be added.
    I don't understand where the heck the formula comes from :confused:
    Can you help me with this problem ?
  2. jcsd
  3. Sep 6, 2004 #2
    1 * 0.75 = (n+1) * 0.15 seems like a more understandable form to me.

    basically, above sais that the (absolute) ammount of alcohol is the same in both mixtures.
  4. Sep 6, 2004 #3
    That formula comes from this one--a more general form:


    Where C is concentration and V is volume.

    Now, using you original numbers I can do this:


    Which can be rewritten into the form you first asked about:


    Hope this helped. Good luck.
  5. Sep 6, 2004 #4

    It clears things up, but what is [tex] c_2 [/tex] ?
  6. Sep 6, 2004 #5
    C2 is 0% because pure water has 0% alcohol.
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