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 swims downstream 48 km and upstream 42 km taking 6 hours each time ; what is the speed of the current ?
0.5 kmph
2 km/h.