Two piers, a and b, are located on a river: b is 1500 m downstream from a (figure 1) . two friends must make round trips from pier a to pier b and return. one rows a boat at a constant speed of 4.00 km/h relative to the water; the other walks on the shore at a constant speed of 4.00 km/h. the velocity of the river is 2.80 km/h in the direction from a to b.