A boat takes 3hr to travel 30 km down a river, then 5hr to return. How fast is the river flowing?

Answer 1

Let V = stream velocity and v = boat velocity with respect to water. Both are in km/hr.

3 hr = 30/(V + v)
5 = 30/(V -v)

Solve those two equations in two unknowns for V and v.
5(V -v) = 3(V +v)
2V = 8v
v = V/4

3 = 30/(5V/4)= 120/5V
15V = 120

etc