Sometimes a figure can be transformed completely by moving very little. Here, three coins are enough—provided you find the exact positions.
The coin triangle
Riddle statement
Ten coins form the triangle shown.
Move exactly three coins to make another equilateral triangle of the same size, but inverted, with its point facing downward.
You may not add, remove, or overlap coins.
Show solution
Solution
Answer: move the coin at the top point and the two coins at the ends of the base.
Key idea: the inverted triangle can overlap the original in seven positions. Those seven coins remain fixed, so only three missing positions need to be filled.
The three moved coins go to:
- the two ends of the new top row;
- the new bottom point.
The final triangle is the same size and points downward. The challenge is seeing the overlap, not moving many coins.