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 man can row upstream at 27 kmph and downstream at 35 kmph , and then find the speed of the man in still water ?
31
(35 + 27) / 2 = 31 kmph.