The answer, it turns out, is 144 — and the formula used to get to that answer is what's now known as the Fibonacci sequence. "Liber Abaci" first introduced the sequence to the Western world.

The sequence begins with 0 and 1, with each subsequent number being the sum of the previous two according to the formula x n = (x n −1) + (x n −2): The power of the Fibonacci sequence lies in ...