Problem 7: Two friends are travelling downstream on a raft drifting along with the same speed as the river. At t = 0, Friend-1 jumps off the raft and start swimming downstream. At t = 30 min, Friend-1 turns around and starts swimming upstream until he reaches the raft. By this time the raft has drifted 2 km downstream. Friend-1 always swims at a constant speed relative to the river. a) Find the speed of the river. Ans: 2 km/h b) How does your answer to part a) depend on the Friend-1's speed relative to the river?