How to set up this algebra word problem

Question

Professor Baker wants to mix a solution containing 15% acid with one containing 20% acid to obtain a 25-ounce solution containing 16% acid.  How many ounces of the 15% solution should the professor use?

Answer 1

Let x = the number of ounces of 15% acid

Let y = the number of ounces of 20% acid

x + y = 25      y = 25 - x

.15x + .20y = .16(25) = 4

.15x + .20(25-x) = 4        Multiply both sides of the equation by 100 to get rid of the decimal points.

15x + 20(25-x) = 400

15x + 500 - 20x = 400

-5x = -100

x = 20 ounces