Okay i'm totally stuck and nobody I know really gets it either, so i've turned to Yahoo for help :)
Basically there are four aliens on a planet. They want to go on a sea-saw. The end seats on the sea-saw are 2m from the pivot, and the middle seats are 1m from the pivot. The Aliens way 400N, 300N, 200N and 100N - which two can sit on the sea-saw (on any of the seats) and make it balance? It'd be a huge help if you at least help me work out the answer! :)

Here is the rule for see-saws here on Earth, and there is no reason to expect that it doesn't work exactly the same anywhere else:                     (weight) x (distance from the pivot) on one sideis equal to                     (weight) x (distance from the pivot) on the other side.That's why, when Dad and Tiny Tommy get on the see-saw, Dad sitscloser to the pivot and Tiny Tommy sits farther away from it.       (Dad's weight) x (short length) = (Tiny Tommy's weight) x (longer length).So now we come to the strange beings on the alien planet.There are three choices right away that both work:#1).(400 N) in the middle-seat, facing (200 N) in the end-seat.        (400) x (1)  =    (200) x (2)#2).(200 N) in the middle-seat, facing (100 N) in the end-seat.       (200) x (1)  =    (100) x (2)#3).On one side:  (300 N) in the end-seat       (300) x (2) = 600On the other side:                      (400 N) in the middle-seat  (400) x (1) = 400           and     (100 N) in the end-seat      (100) x (2) = 200                                                    Total . . . . . . . . . . . . 600  These are the only ones to be identified at Harvard . . . . . . .There may be many others but they haven't been discarvard.

400N and 100N on one side and 300N and 200N on the other side