This is a number balance.
It is also called a 'balance bar' or an 'equaliser'. It has weights:
These are hung below the numbers. It balances equal numbers, for example like this:
Where would you need to hang the weight to make the one below balance?
What about this one?
If you had to use two weights and make the one below balance, where could you put them?
How many different ways can you make it balance with two weights?