Answer:
The answer is 4 liters.
Step-by-step explanation:
We want a final 25% solution, which means that the solvent should constitute 25 percent of all the liters of solution.
Let us call [tex]x[/tex] the litres of 40% solution, so when they are added to the 12 litres of 20% solution, the total litres of the new solution are:
[tex]x+12[/tex]
of these liters, 25% must be the solvent:
[tex]N=0.25(x+12)[/tex]
Now for the 40% solution, the liters of the solvent are [tex]0.4x[/tex], and for the 20% liter solution, the liters of solve are [tex]0.2*12=2.4[/tex] liters. Thus the total liters [tex]N[/tex] of the solvent are
[tex]N=0.4x+2.4[/tex]
Therefore
[tex]N=0.4x+2.4=0.25(x+12)[/tex]
[tex]\therefore x=4\:litres[/tex]
