All the coins look the same. One of them is fake - it is lighter than all the others.
At the top you see all the coins of the task side by side, each with its number. Below are the weighings that have already been made.
The pan that hangs lower is the heavier one. The fake coin is light, so it can never be in the lower pan.
If a scale is level, all the coins on it weigh the same - so they are all real. A level scale tells you the most.
Finally, write the number of the fake coin in the box.
Example
For parents
What does this exercise practice?
Nothing is calculated here, things are concluded. The child practices drawing a conclusion from an observation and ruling out possibilities until only one is left. On top of that comes comparing weights - heavier, lighter, equal - entirely without numbers. That is exactly the kind of thinking needed later for sorting, measuring and checking results.
Common pitfalls
The most common mistake is to overlook the level scale. Children look where something moved and forget that balance is the strongest piece of information - it wipes out every coin on both pans at once. With 5 coins and 1 and 2 against 3 and 4 balancing, only 5 is left, without any second weighing. The second stumbling block is the direction - the fake coin is lighter, so its pan goes up, not down.
How to practice at home
Take nine identical coins and secretly swap one for a lighter one, for example a button of the same size. Put three in each hand and compare. Who can find the light coin with only two comparisons? You do not even need a scale - two hands make a good balance.
Related: Which Scale Fits?, Logic grid