These "mixture problems" can be solved if we organize our facts.How many liters of a 25% alcohol solution must be mixed with a 12% solution
to get 13 liters of a 15% solution.?
Let = liters of 25% solution.
Then = liters of 12% solution. (I hope you see why.)
The liters of 25% solution contains: . liters of alcohol.
The liters of 12% solutions contains: . liters of alcohol.
. . Hence, the mixture will contain: . liters of alcohol. .
But we know that the final mixture will be 13 liters which is 15% alcohol.
. . It will contain: . liters of alcohol. .
We just described the final amount of alcohol in two ways.
There is our equation! . . . Equate  and 
. . . .