A submarine can travel a certain distance downstream in 4 hours and the same distance upstream in 8 hours. If the submarine's speed in still water is 12 km/h, what is the speed of the stream?

ATQ, Ratio of time taken by the submarine to cover the same distance in downstream and upstream = 4:8 = 1:2 So, ratio of upstream speed to downstream speed, of the boat = 1:2 (Since, distance covered is same therefore, ratio of speeds will be inverse of the ratio of time taken) Let the upstream speed of the submarine be 'a' km/h So, downstream speed of the Submarine = a × (2/1) = '2a' km/h ATQ; Speed of Submarine in still water = {(upstream speed + downstream speed)/2} So, {(2a + a)/2} = 12 Or, 3a = 24 So, a = 8 So, speed of the stream = speed of Submarine in still water - upstream speed = 12 - 8 = 4 km/h