Quote from : The Maze of Bones Book

I've got a few ideas," (Amy) admitted. "But I don't know where we're going in the long term. I mean - have you ever thought about what this ultimate treasure could be?" "Something cool." (Dan) "Oh, that's real helpful. I mean, what could make somebody the most powerful Cahill in history? And why thirty-nine clues?" Dan shrugged. "Thirty-nine is a sweet number. It's thirteen times three. It's also the sum of five prime numbers in a row - 3,5,7,11,13. if you add the first three powers of three, 3 to the first, 3 to the second, and s to the third, you get thirty-nine." Amy stared at him. "How did you know that?" "What do you mean? It's obvious.