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: how many liters of a 40 % iodine solution need to be mixed with 35 liters of a 20 % iodine solution to create a 35 % iodine solution ?
105
15 liters.