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 side
is 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 sits
closer 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) = 600

On 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.