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 can do a work in 9 days and b can do the same work in 18 days . if they work together , in how many days will they complete the work ?
6 days
6 days.