Question

10. Sue has a boat that would go 9 miles per hour in still water. She travels downstream for a

certain distance and then back upstream to where she stared. Sue notices that it takes her 4
hours to travel upstream against the river, and only 2 hours to travel downstream with the
river. The rivers speed is r miles per hour. Write and Solve for r.

Answer 1

The speed of the river is 3 miles per hour.

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).

`(9 - r)/(9 + r) = (2)/(4) \\18 - 2r = 9 + r\\3r = 9\\r = 3`