The speed of a boat in still water is 20 km/hr, and the speed of the stream is 5 km/hr. The boat takes 8 hours more to cover D km upstream than to cover the same distance downstream. Find the time taken by the boat to cover D km in still water.

ATQ, Speed of boat in still water = 20 km/hr Speed of stream = 5 km/hr Downstream speed = 20 + 5 = 25 km/hr Upstream speed = 20 − 5 = 15 km/hr Let the distance be D km. Time taken upstream = D/15 hours Time taken downstream = D/25 hours According to the question, upstream time is 8 hours more than downstream time. So, D/15 − D/25 = 8 LCM of 15 and 25 = 75 D/15 = 5D/75 D/25 = 3D/75 So, 5D/75 − 3D/75 = 8 2D/75 = 8 D = 8 × 75/2 D = 300 km Now, time taken to cover D km in still water = Distance/Speed in still water Time = 300/20 = 15 hours