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 is 30 % more efficient than b . how much time will they working together take to complete a job which a alone could have done in 23 days ?
13
11.76 days.