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we have two mixtures: one contains 7% acid and the other one contains 4% acid. How much do we combine of each to the two available mixtures to get 300 ml of 5% mixture.
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It is a typical mixture word problem.
See the lesson Solving typical word problems on mixtures for solutions in this site, Problem 1.
This problem is very similar to yours.
In this lesson you will find the detailed solution.
Consider it as a sample. Read it attentively.
Then solve your problem by substituting your data.
In this way you will learn how to solve problems like this, once and for all.