Answer:
The speed of the river is 3 miles per hour.
Step-by-step explanation:
In downstream, the speed of the boat will be (9 + r) miles per hour and in upstream the speed of the boat will be (9 - r) miles per hour.
Ratio of the speed in downstream to the speed in upstream is (9 + r):(9 - r).
Hence, the ratio of the time taken by the boat to cover the distance in downstream to upstream is (9 - r):(9 + r).
[tex]\frac{9 - r}{9 + r} = \frac{2}{4} \\18 - 2r = 9 + r\\3r = 9\\r = 3[/tex]
