A boat travels 30 km downstream in 2.5 hours and returns the same distance upstream in 3.75 hours. What is the boat's speed in still water?

Explanation

The downstream speed is 30 km ÷ 2.5 hr = 12 km/h. The upstream speed is 30 km ÷ 3.75 hr = 8 km/h. The boat's speed in still water is the average of these two speeds: (12 + 8) ÷ 2 = 10 km/h, which is not listed among the options.