Question

Pure water is being added to a 25% solution of 120 milliliters of hydrochloric acid. How much water should be added to reduce it to a 15% mixture ?

Answer 1

here's one way. You want the concentration to be reduced by a factor of 15/25 = 3/5

So, the volume must be increased by a factor of 5/3.

5/3 * 120 = 200 ml
So, if you add 80ml of water, the concentration is reduced from 25% to 15%

Or, you can solve the equation

.25*120 + 0x = .15(120+x)

and arrive at the same answer.