Relative Velocity Homework: Solving Round Trip Time

[casemeister06]

Homework Statement

Two piers, A and B are located on a river: B is 1500 m downstream from A. Two friends must make round trips from pier A to B and back. One rows a boat at a constant speed of 4 km/hr relative to the water; the other walks on the shore at a constant speed of 4 km/hr. The velocity of the river is 2.80 km/hr in the direction from A to B. How much time does it take each person to make the round trip.

Homework Equations

V_avg = $$\Delta x / \Delta t$$

V_P/A = V_P/B + V_B/A

The Attempt at a Solution

I'm assuming we would have to find a single average velocity for both the boat and the person then take each velocity and let it be the dividend of the total distance to solve for the time. The person is relatively easy to solve for. For the boat though. I'm having trouble setting up the relative velocity equation. I get confused on what my point of reference should be and if the speeds are with respect to the water or the earth.

 

[jbsteele]

Person: Vavg = \Delta x / \Delta tVavg = 1500 m / \Delta t\Delta t = 1500 m / 4 km/hr\Delta t = 0.375 hrBoat:VP/A = VP/B + VB/AI'm not sure how to set this up.