Question

A boat takes 3.0hr to travel 28km down a river, then 6.0hr to return. How fast is the river flowing?

Answer 1

Let
v = speed of boat with respect to river
V = speed of river with respect to shore
t = time in downstream journey
T = time in upstream journey
s = distance

Then you'll get two equations with two unknowns -
(v+V) = s/t
(v-V) = s/T

Solve for V

Answer 2

Once again, this question requires the student to make an assumption. The assumption is that the boat moves at a constant speed through the water. This should have been stated in the question, but otherwise the above response is correct.