Your trip to find all the necessary ingredients for this cake is taking much longer than you realize, but after a lot of trekking, swimming, spelunking and a considerably large medical bill, you and Pinkie have arrived at what she assures you is the final stop, and then she'll have everything she needs to finally start baking the cake. With you, obviously, because you're smart. And she gives you all your precious pecan muffins. And she refuses to go down alone if caught.
You appear to have arrived at a strange temple, where an extremely wizened old pony appears to be standing guard in front of a series of stepping stones across a pond. He is apparently the keeper of the final ingredient that you need: 50M8RA extract, and although Pinkie isn't entirely sure what a 50M8RA even is or what was extracted from it (although you have a sinking feeling you know) only those the keeper deems worthy can have a cup of it. Worried, the pai of you approach the keeper, who goes though the expected spiel of how 'none may pass' and you must 'answer his riddle or die', and all that. Before you can even protest, Pinkie announces you'll do it. Jeez, what happened to manners nowadays? It's dead, dude.
Anyway, you've been shoved in the spotlight, so it's time for you to make a stand. The keeper gestures to the pond, explaining that before you is a completely straight line of twenty stepping stones. Starting on the shore of the pond, which he rather helpfully lists as Number 0, and ending precisely on the last stepping stone, which he lists as Number 20, is how anypony who wants to pass to the other side and claim the prize of the 50M8RA extract should go about it.
Pinkie then asks if the puzzle is to cross, and for her dumb question she is promptly beaned on the head with a small rock the keeper had hidden in his robes. Of course that isn't the puzzle: there's a bunch of ways to get across. The puzzle, he says with a mocking grin, is to determine EXACTLY HOW MANY ways there are to cross: to start at the shore and then end on the twentieth stepping stone exactly.
Ah. Well, that's a slightly trickier task to work with, but in your mind, certainly doable. So what's the answer? Pinkie won't be happy if you have to walk home without the last ingredient after all your trekking, swimming, spelunking and medical expenses, so spare yourself the embarrassment and get this solved.