Final answer:
The speed of the river is 1 km/h.
Step-by-step explanation:
To calculate the speed of the river, we need to understand the concept of relative velocity. When the boat is traveling downstream, it is helped by the speed of the river. So, the speed of the boat in still water can be calculated by subtracting the speed of the river from the total speed downstream. In this case, the boat takes 3 hours to travel 27 km downstream. Therefore, the speed downstream is 27 km / 3 h = 9 km/h.
Now, to calculate the speed of the river, we subtract the speed of the boat in still water from the speed downstream. So, the speed of the river is 9 km/h - 8 km/h = 1 km/h.
Therefore, the river is flowing at a speed of 1 km/h.