Alas, The Pointless Things You Can Do With Marbles
Finn has a box containing only red marbles, green marbles, and blue marbles. He needs to select at least seventeen marbles to be guaranteed to be sure that at least one is green. Likewise, he needs to select at least eighteen marbles to be sure at least one is red. To be sure of getting at least one of each color, he must select twenty marbles.
How many marbles are in the bag?