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Word Problem - Please Help -

  1. Jun 26, 2005 #1
    1. A tank holds 90 L of chemical solution. Currently the solution has a strength of 20%. How much of this should be drained and replaced with an 80% solution to have a final strength of 30%?
  2. jcsd
  3. Jun 26, 2005 #2


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    Suppose you "drain and replace" X liters of the 20% solution. That leaves 90- X liters of 20% solution. The X liters of the 80% solution will contain 0.8X L of the chemical. The remaining 90-X liters will contain 0.2(90- X) L of the chemical so you will now have 0.8X+ 0.2(90-X) L of the chemical in the final result. How much chemical will there be in 90 L of 30% solution? Set those equal and solve for X.
  4. Jun 26, 2005 #3
    Answer is 15 L.

    thx :D
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