A boatman rows 2 kilometers against the stream in 1 hour and covers 1 kilometer downstream in 10 minutes. How much time will he require to travel 5 kilometers in still water?

Explanation

The upstream speed is 2 km/h (2 km in 1 hour). The downstream speed is 6 km/h (1 km in 10 minutes equals 6 km/h). The speed in still water is the average of upstream and downstream speeds: (2 + 6) / 2 = 4 km/h. To cover 5 km in still water, time = distance/speed = 5/4 = 1.25 hours, which is 1 hour and 15 minutes.