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4 gallons

Let x be the amount of antifreeze needed to be added.

We know that the total amount of antifreeze in the new solution must equal the amount of antifreeze in the old solution + x:

.40*(x+12)=x+.20*12

.60x=2.40

x=4 gallons

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Q: How much antifreeze must be added to 12 gallons of 20 percent antifreeze to make a 40 percent antifreeze solution?
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Related questions

How much water should be added to 1 gallon of pure antifreeze to obtain a solution that is 60 percent antifreeze?

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How much water should be added to 1 gallon of pure antifreeze to obtain a solution that is 80 percent antifreeze?

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How many gallons of 80 percent antifreeze solution must be mixed with 100 gallons of 10 percent antifreeze to get a mixture that is 70 percent antifreeze?

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