Eli wants to combine 0.5 gallon of a 10% acid solution with some 35% acid solution to make a 15% acid solution. | Which equation can you use to determine how many gallons of the 35% acid solution Eli should add?

The amount of pure acid in the mixture equals the amount of acid in the 0.15-concentration final solution. The 0.5 gallon of 10% solution contributes (0.10)(0.5) gallons of acid, and g gallons of 35% solution contributes 0.35g gallons. The final mixture has g + 0.5 gallons, so its acid amount is 0.15(g + 0.5); therefore, choice A is correct.

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