Explanation:

- The boat covers 5 km upstream in 15 minutes, which is 1/4 hour. Therefore, its upstream speed is 5 km / (1/4) hr = 20 km/h.

- Let the boat's speed in still water be 6x and the speed of the stream be x.

- Upstream speed = (6x - x) = 5x. We know the upstream speed is 20 km/h, so 5x = 20. Thus, x = 4 km/h.

- The speed of the boat in still water is 6x = 24 km/h.

- The downstream speed = (6x + x) = 7x = 28 km/h.

- To cover 19.6 km downstream: Time = Distance/Speed = 19.6 km / 28 km/h = 0.7 hours.

- Convert 0.7 hours into minutes: 0.7 × 60 = 42 minutes.

Correct Answer: 42 minutes

“”