The speed of a boat in still water is 9 km/hr and the speed of the stream is 6 km/hr. If the boat covers a round trip journey in 6 hrs, then the time taken by the boat to complete the downstream journey is

Given:

The speed of the boat in still water = 9 km/hr

The speed of the stream = 6 km/hr

Total time for the round trip = 6 hrs

Formula used:

Speed downstream = (speed of boat in still water + speed of stream)

Speed upstream = (speed of boat in still water - speed of stream)

Time = Distance / Speed

Calculations:

Speed downstream = 9 + 6 = 15 km/hr

Speed upstream = 9 - 6 = 3 km/hr

Let the distance of one leg of the journey (one-way distance) be D km.

Total time for the round trip = Time downstream + Time upstream

6 = (D / 15) + (D / 3)

⇒ 6 = (D / 15) + (5D / 15)

⇒ 6 = (6D / 15)

⇒ D = (6 × 15) / 6

⇒ D = 15 km

Time taken for the downstream journey = D / Speed downstream

⇒ Time = 15 / 15

⇒ Time = 1 hr

∴ The correct answer is option (3).