A boat covers 100 km downstream in 10 hours and travels 75 km upstream in 15 hours. What is the speed of the current?

Explanation

The downstream speed is 100 km ÷ 10 hours = 10 km/h, and the upstream speed is 75 km ÷ 15 hours = 5 km/h. The speed of the stream is half the difference between downstream and upstream speeds: (10 - 5) ÷ 2 = 2.5 km/h.