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 ?

An athlete crosses a 23 m wide river by swimming perpendicular to the water current at a speed of 0.85 m/s relative to the water. He reaches the opposite side at a distance 41 m downstream from his starting point. Randomized Variables w = 23 m d=41 m Vs = 0.85 m/s