How many ounces of 30% hydrochloric acid solution and 80% hydrochloric acid solution must be mixed to get 10 oz of 50% hydrochloric acid solution

I like to use a little X diagram to work mixture problems. The constituents are listed on the left, the mix value in the middle, and the differences are listed on the right along the lines of the X.

This diagram tells you the constituents 80% and 30% solution are in the proportion 20:30 or 2:3 or 4:6 to make the mix having 50% hydrochloric acid. Note that the numbers 4 and 6 add to 10, the number of ounces of solution we want.

4 ounces of 80% solution must be mixed with 6 ounces of 30% solution to make 10 ounces of 50% solution.

The correct answer is addition