What is the relative velocity of a man swimming downstream with speed 12 km/h (in still water) with respect to a child running towards the river with speed 4 km/h in direction perpendicular to water flow. Speed of water flow = 4 km/h.
The relative speed of the man with respect to river = 12 + 4 = 16 km/hr.
But since the speed of the child is perpendicular to water flow, the relative velocity of the man with respect to child is 0.