physics
posted by john .
Two boats start together and race across a 42 km wide lake and back. Boat A goes across at 42 km/h and returns at 42 km/h. Boat B goes across at 21 km/h, and its crew, realizing how far behind it is getting, returns at 63 km/h. Turnaround times are negligible, and the boat that completes the roundtrip first wins.
(a) Which boat wins? (Or is it a tie?)
boat A
boat B
it's a tie
By how much?
km
(b) What is the average velocity of the winning boat?
km/h
I have no idea how to do this please help!

I am very confused with this problem and as to how the answer is found but the correct answer is the following
boat a wins
by 42km
and the average velocity of the winning boat is 0(zero). I got it right by practicing a webassign problem.