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 pier b and return. one rows a boat at a constant speed of 4.50 km/h relative to the water. the other walks on the shore at a constant speed of 4.50 km/h. the velocity of the river is 2.60 km/h in the direction from a to b. how much time does it take each person to make the round trip?