Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: a chemist mixes one liter of pure water with x liters of a 40 % salt solution , and the resulting mixture is a 20 % salt solution . what is the value of x ?
1
0.5 liters.