You might like to have a look at Number Balance before trying this problem.

Here is a number balance, or equaliser:

Full Screen Version
This text is usually replaced by the Flash movie.

It has weights hanging off it to make it balance.
We can hang more than one weight from each hook.
I hang one weight on the $10$. I want to use just one hook on the other side of the equaliser.
Where could I put weights and how many would I need to put on?
Now I hang two weights on the $10$ on one side of the equaliser.
Using only the $2$ hook, how could I add weights on the other side to make it balance?
Using only the $5$ hook, how could I add weights on the other side to make it balance?
How could you make it balance using both the $2$ and the $5$ hooks? Is there only one solution?
Now I hang one weight on the $10$ hook on one side of the equaliser.
If I use the $3$ hook and one other, how can I make it balance?