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: 40 liters of a mixture is created by mixing liquid p and liquid q in the ratio 5 : 3 . how many liters of liquid q must be added to make the ratio 5 : 6 ?
15
10 liters.