Okay, first let's convert the 400 kg to a force by multiplying it times 9.8 m/s/s, which is the force of gravity:
400 kg x 9.8m/s/s = 3920 N of force
Then lets use the formula for work, which is work= force x distance:
39,200 Nm, or J= 3920 N x 10 m
Last, but not least, let's use the formula for power, which is power=work/time:
4900 J/s = 39,200 J/8 seconds
So the correct answer is 4900 Joules per second, or Watts, which is the unit for power.
It should be noted that power is the amount of work done per unit of time. Things that have more power can do more work in less time; that is why they are "powerful"!