I think the answer maybe be 10 miles. If he rowed upstream, that is four, and it took only one hour. Downstream he went 6 mph. We dont know how long he spent each way, so it might be cut evenly in half. Saying he spent one hour too and fro, the answer would be ten because all we have to do is add 4 and 6. This may be completely wrong, but there isn’t that much context.
