The value of the swimmer's power output is calculated by dividing the work done by the time it took for the work to be completed. From the given in this item,
P = 3560 J/ 55 s = 64.73 W
Rounding off to two significant figures will give us 65 W.
The correct order is (in decreasing order of gravity strength)
Jupiter - Neptune - Venus - Mars
In fact, Wayne's weight on each planet is given by

where m is Wayne's mass, which is a constant value, and g is the gravity strength at the surface of the planet. Therefore, the Wayne's weight W on each planet is directly proportional to the gravity strength of that planet: so the planet with the strongest gravity is the one where Wayne's weight is the greatest (Jupiter, 333 pounds), followed by Neptune (159), Venus (128) and Mars (53).
The speed of the ball is always zero and the acceleration is always -g when it reaches the top of its motion. This is because when the ball is free, only gravity acts on it which is always downwards, hence g is the net acceleration and it is always negative. However the velocity does not direction change instantly, negative acceleration first slows down the ball with a positive velocity, until that point the ball keeps moving up, then the ball velocity becomes zero just before changing direction and becoming negative after which the ball will now go down along gravity. Hence the ball velocity is zero at the top (neither going up nor down). Mathematically this can be seen as velocity is the integration of acceleration.