Short Answer 
The boat’s downstream speed is 6 km/h and upstream speed is 5 km/h, giving an average speed of 5.5 km/h in still water. To travel 55 km at this speed, the time taken is 10 hours, which converts to 6 hours and 10 minutes.
Determine Boat Speeds
To calculate the time taken by the boat to travel in still water, we first need to find the boat’s speed in both downstream and upstream currents. We have:
- Downstream speed: 54 km in 9 hours = 6 km/h
- Upstream speed: 50 km in 10 hours = 5 km/h
Calculate Average Speed
Next, we calculate the average speed of the boat in still water. Since the downstream speed is greater than the upstream speed, we can determine the still water speed by averaging the two speeds:
- Average speed = (6 km/h + 5 km/h) / 2 = 5.5 km/h
Calculate Time to Travel Distance
Finally, we use the average speed to find out how long it takes to travel a distance of 55 km in still water. We apply the formula for time:
- Time = Distance / Speed = 55 km / 5.5 km/h = 10 hours
Converting 10 hours results in 6 hours and 10 minutes as the final answer.