system of eqns: 4%, 12% solns mixed to get 12 oz 9% soln

van said:

Alyssa is a chemist. She needs to prepare 12 ounces of a 9% hydrochloric acid solution. Find the amount of 4% and the amount of 12% solution she should mix to get this solution.

so far i only have:

let x = the amount of 4% solution. <<< How much acid is here .......................(1)

let y = the amount of 12% solution.<<< How much acid is here .......................(2)

x + y = the amount of 9% solution.<<< How much acid is here .......................(3)

By rule of mixture:

amount of acid in (1) + amount of acid in (2) = amount of acid in (3)

my teacher told us this: "amount of solution * __% = amount of acid"

at this point, i am stuck as to where to even start.

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