Triangular Number Order
Ten is a triangular number, because 10 objects can be neatly arranged in an equilateral triangle.
In general, a triangular number is a number that can be represented as a triangle with one object in the first row and each subsequent row contains one more element than the previous row. (From the picture above, you might notice that 1, 3, and 6 are also triangular numbers.)
Order the digits 1 through 9 so that the sum of any two adjacent numbers is a triangular number.
This brainteaser was written by Derrick Niederman.
