I believe it is 4.5mph because traveling upstream and the stream moves at a speed of 4mph would knock off 4 of the original number so if you add 4miles to the given 5 miles you get 9
The same is with the boat going downstream it is adding 4 miles to the original number so you subtract 13 and 4 and you get 9
If it took 2 hours to do that then you divide 9 by 2 and get 4.5
