A motorboat’s speed in still water is 10 km/hr, and the speed of the stream is 2 km/hr. The boat takes 5 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 = 10 km/hr Speed of stream = 2 km/hr Downstream speed = 10 + 2 = 12 km/hr Upstream speed = 10 − 2 = 8 km/hr Let the distance be D km. Time taken upstream = D/8 hours Time taken downstream = D/12 hours According to the question, upstream time is 5 hours more than downstream time. So, D/8 − D/12 = 5 LCM of 8 and 12 = 24 D/8 = 3D/24 D/12 = 2D/24 So, 3D/24 − 2D/24 = 5 D/24 = 5 D = 5 × 24 = 120 km Now, time taken to cover D km in still water = Distance/Speed in still water Time = 120/10 = 12 hours