You need 250 mL of a 90% alcohol solution. On hand, you have a 75% alcohol mixture. How much of the 75% alcohol mixture and pure alcohol will you need to obtain the desired solution?

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Step-by-step explanation:

Let x represent the quantity (in mL) of pure alcohol needed for the mix. Then the amount of 75% needed is (250-x). The amount of alcohol in the mixture is ...

  1.00x +0.75(250 -x) = 0.90(250)

  0.25x +187.5 = 225 . . simplify

  0.25x = 37.5 . . . . . . . . subtract 187.5

  x = 150 . . . . . . . . . . . . . divide by 0.25

  (250 -x) = 100 . . . . mL of 75% solution

You need 100 mL of the 75% solution and 150 mL of pure alcohol to obtain the desired mixture.

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