Two solutions of different concentrations of acid are mixed creating 40 mL of a solution that is 32% acid. One-quarter of the solution is made up of a 20% acid solution. The remaining three-quarters is made up of a solution of unknown concentration, c.Which equation can be used to determine c, the unknown concentration?

One-quarter of 40 mL is 10 mL, so 10 mL is the 20% solution. The remaining three-quarters is 30 mL at concentration c. The total amount of acid is therefore 10(0.2) + 30c, and it must equal the acid in 40 mL of a 32% solution, 40(0.32). This gives 30c + 10(0.2) = 40(0.32), choice A.

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