Dione carries a 20kg box to the top of a 30m tall building. Only then does he realize that it actually belongs on the sixth floor, which is exactly halfway up the building. After he delivers the box to the sixth floor, how much work has he done on the box?
20 * g * h = 20 * 9.81 * 30/2 = 2943 Joules
Note, the thing about work and potential energy is that it does not matter what path you take from the start to the finish, only the difference in potential (in this case height) between start and finish. In the top half of the building you do exactly as much work upward as the box does on you coming down, so the top half of the building is not part of the answer.