An athlete crosses a 28 m-wide river by swimming perpendicular to the water current at a speed of 0.8 m/s relative to the water. He reaches the opposite side at a distance 43 m downstream from his starting point.
a) How fast is the water in the river flowing with respect to the ground in m/s?
b) What is the speed of the swimmer with respect to a friend at rest on the ground in m/s?