How many liters of an 18% alcohol solution must be added to a 10% alcohol solution to get 20 L of a 15 alcohol solution

You can set this up with one or two equations. I prefer one... less work.

Let x=the amount of 18% to be added.

Then, the amount of 10% is what remains of the 20 L not taken up by the 18%, so we have 20-x.

So, we have

Solve for x.

See how to do these now.