The current in a stream moves at a speed of 4 mph. A boat travels 5 miles upstream and 13 miles downstream in a total time of 2 hours. What is the speed of the boat in still water?

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