Two boats start together and race across a 56 km wide lake and back. Boat A goes across at 56 km/h and returns at 56 km/h.

Two boats start together and race across a 56 km wide lake and back. Boat A goes across at 56 km/h and returns at 56 km/h. Boat B goes across at 28 km/h, and its crew, realizing how far behind it is getting, returns at 84 km/h. Turnaround times are negligible, and the boat that completes the round-trip first wins. what is also the avg velocity of the winning boat. Which boat wins and by how much?