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A health clinic uses a solution of bleach to sterilize petri dishes in which cultures are grown. the sterilization tank contains 120 gal of a solution of 4% ordinary household bleach mixed with pure distilled water. new research indicates that the concentration of bleach should be 7% for complete sterilization. how much of the solution should be drained and replaced with bleach to increase the bleach content to the recommended level?

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sqdancefan
Let g represent the number of gallons to be drained. Then the amount (in gallons) of bleach in the tank will be given by
.. 4%*(120 -g) +100%*g = 7%*120
.. 4.8 -.04g +g = 8.4
.. 0.96g = 3.6
.. g = 3.6/0.96 = 3.75

3 3/4 gallons need to be drained and replaced with 3 3/4 gallons of bleach.

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