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 work as fast as b . if b can complete a work in 36 days independently , the number of days in which a and b can together finish the work in ?
12 days
18 days.