Undo it backwards

Find out how many weights are hidden in one bag. Every bag holds the same number, called x, and right now the balance is level. Press a move. A fair move keeps the balance level. An unfair move tips it, and you have to put things back.

What's going on

The balance is a picture of an equation. The left pan is one side, the right pan is the other, and "level" means "equal". The equation 3x + 4 = 19 says: three bags and 4 loose weights weigh the same as 19 weights.

Somebody made the left side in two steps. They took x, multiplied it by 3, then added 4. To get back to x, undo those steps in the opposite order, the way you take off shoes before socks. First undo the adding: take 4 from both sides. Then undo the multiplying: share both sides into 3 equal groups and keep one.

The one rule is to be fair. Whatever you do to one pan, do to the other. Try the two unfair buttons on purpose and see which way the balance tips.

Sheet: Undo it backwards