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 3 days . b can do in 6 days . if both a & b are working together in how many days they can finish the work ?
2
1.5 days.