A boatman travels 2 km upstream against the current in 1 hour and covers 1 km downstream with the current in 10 minutes. How much time will he require to travel 5 km in still water?

Explanation

Given the boatman's speeds upstream and downstream, we can calculate the speed in still water. The time taken to cover 2 km upstream is 1 hour, so upstream speed = 2 km/h. The time to cover 1 km downstream is 10 minutes (1/6 hour), so downstream speed = 6 km/h. The speed in still water is the average of upstream and downstream speeds: (2 + 6)/2 = 4 km/h. To travel 5 km in still water at 4 km/h, time = distance/speed = 5/4 = 1.25 hours, which is 1 hour 15 minutes.