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 and b finish the job in 15 days . while a , b and c can finish it in 11 days . c alone will finish the job in
41.25 days
7 days and a half.