How to play The Heavy Coin
- Choose 3, 9 or 27 coins.
- Click coins to put the same number on each pan, then press Weigh.
- Use what the scale tells you to rule coins out.
- When you know the heavy coin, press I know which coin it is and click it.
Maths skills this game builds
- Problem solving and strategy
- Dividing into equal groups
- Powers of 3 (3, 9, 27)
- Reasoning from evidence
Tips for parents and carers
- Try the 9-coin puzzle and ask: what is the fewest weighings you think you need?
- Once your child finds the strategy, try the 27-coin puzzle. The same idea works again.
- Try it with objects at home, such as 9 identical small items with one hidden heavier.
Using The Heavy Coin with a group or class
- Let groups plan their first weighing together before anyone clicks. Ask: What will we know after one weighing, whatever happens?
- Link to powers of 3: each weighing has three possible results, so one weighing sorts coins into three groups.
- Online with up to 12 children: children can draw circles on paper and cross them out as they are ruled out.
Frequently asked questions
What is the fewest number of weighings?
3 coins need 1 weighing, 9 coins need 2, and 27 coins need 3.
Why do you split the coins into three groups?
Because a balance can give three answers: left heavier, right heavier or balanced. Each answer points to one group.
Is the heavy coin always in a different place?
Yes. A new puzzle starts with a randomly chosen heavy coin.