Two boats start together and race across a 40-km-wide lake and back. Boat A goes across at 40 km/h and returns at 40 km/h.

Two boats start together and race across a 40-km-wide lake and back. Boat A goes across at 40 km/h and returns at 40 km/h. Boat B goes across at 20 km/h, and its crew, realizing how far behind it is getting, returns at 60 km/h. Turnaround times are negligible, and the boat that completes the round-trip first wins.