Inverting the 10-Coin Triangle Puzzle and Other Shapes: A New General Solution

Abstract

This puzzle, often called the "Reverse the Triangle Puzzle," appears regularly in puzzle books. Four rows consisting of 1 coin in row 1, 2 coins in row 2, 3 coins in row 3, and 4 coins in row 4 form the shape of a triangle. What is the fewest number of coins you need to move to flip the triangle 180 degrees so that it points in the opposite direction? In the case of 4 rows, it requires moving just 3 coins. The general solution for any number of rows can be calculated by dividing the number of coins by 3 and ignoring any remainder. A triangle of 4 rows has 10 coins, so 10/3 = 3.333 leads to 3 moves. However, this calculation is fairly nonintuitive as to why it works and requires an inelegant maneuver of periodically discarding a remainder. In this paper, we present a more intuitive formula that always calculates the answer directly and never produces a decimal answer. Further, we explore inverting other shapes (e.g., rhombus) and find an interesting connection to the formula for triangles.