Question

How much pure water must be mixed with 10 liters of a 25% acid solution to reduce it to a 10% acid solution?

Answer 1

How much pure water must be mixed with 10 liters of a 25% acid solution to reduce it to a 10% acid solution?

15L

Answer 2

The solution is 25% acid, which means 2.5 liters acid (10 x .25). 
you need that 2.5 liters to be 10%, or 1/10th the total. 
the total would have to be 25 liters (2.5 x 10) so when you add to 10 to get 25 you get 15 liters. Hope I helped!