Question

A 9 percent salt-water solution is mixed with 4 ounces of an 18 percent salt-water solution in order to obtain a 15 percent salt-water solution. How much of the first solution should be used?
1 ounce
2 ounces
3 ounces
4 ounces

Explain your answer.
let the amount to be added be x ounces

.09x + .18(4) = .15(x+4)

I suggest multiplying by 100, then it is easy to solve for x

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