Both sides weigh the same. Do the same thing to both sides and it stays level.
x + 3 = 7
How can you get one bag on its own?
Each time it balances, the equation is written here.
An equation says two amounts are equal, just like a level balance. Whatever you do to one side, you do to the other, so it stays level. Children see why the rule works before they write it down.
The first level only balances blocks, so children of about 8 can start there. The later levels cover the one-step and two-step equations most children meet between 10 and 14.
A mystery bag is the unknown number x, a block is 1 and a balloon is −1. A block and a balloon together make 0, which shows how a negative number cancels out.
The working is set out like a column sum: the step under both sides, a line, then what is left. It is the layout used in school, so moving from the scale to pen and paper feels natural.