Let

x = gallons of a 25% antifreeze solution

y = gallons of a 10% antifreeze solution

Now, we can writ the following system of equations:

x + y = 15

0.25x + 0.10y = 0.22*15

........

Click here to see the step by step solution for the system of equations

........

x = 12 gal

y = 3 gal

12 gallons for a 25% antifreeze solution and 3 gallons of a 10% antifreeze solution must be mixed.