Two boats start together and race across a 68-km-wide lake and back. boat a goes across at 68 km/h and returns at 68 km/h. boat b goes across at 34 km/h, and its crew, realizing how far behind it is getting, returns at 102 km/h. turnaround times are negligible, and the boat that completes the round trip first wins.

Boat speed is 68km/h both to and fro.
boat b speed to is 34km/h and back is 102km/h
time taken by boat to is distance/speed= 68 km/68km/h=1 hour
and fro is 68km/68km/h=1 hour
for the whole trip boat a took 2 hours
boat b speed to is 34km/h and distance is 68km, so it took 68km/34km/h = 2 hours to and fro it took 68km/102km/h=0.67 hours
for the whole trip boat b took 2.67 hours
so boat a is the winner as it took 2 hours to complete the challenge.