I have a word problem that I know the answer is supposed to be 5 liters, however, that is not the answer I'm getting when I solve it. Can anyone help?

Word problem: It is necessary to have a 40% antifreeze solution in the radiator of a certain car. The radiator now holds 20 liters of 20% solution. How many liters of this should be drained and replaced with 100% antifreeze to get the desired strength? (Hint: the number of liters drained is equal to the number of liters replaced.)

This is the formula I am using, of course, maybe this is my problem.

.20 (20) - .20x + 1.0 (x) = .40 (20)

Please help!