Imagine a triangle of coins on a table so that the first row has one coin in it and the second row has two coins in it and so on. If you can only move one coin at a time, how many moves does it take to make the triangle point the other way?
For a triangle with two row it is one, for a triangle with three rows it is two, for a triangle with four rows it is three.
For a traingle with five rows is it four?