Two students are canoeing on a river. While heading
upstream, they accidentally drop an empty bottle overboard. They
then continue paddling for 2 hours, reaching a point 2.5 km
farther upstream. At this point they realize that the bottle is
missing and, driven by ecological awareness, they turn around
and head downstream. They catch up with and retrieve the bottle
(which has been moving along with the current) 4.0 km downstream
from the turnaround point. (a) Assuming a constant paddling effort
throughout, how fast is the river flowing? (b) What would the canoe
speed in a still lake be for the same paddling effort?