The following contains spoilers for Ready Player One, both the film and the novel it's based on.
Ready Player One is all about solving puzzles in order to win a video game contest. However, one of those puzzles isn't given much attention, which is too bad, because it's really fun to solve. The vast majority of the film's runtime is given over to solving the riddle of the Jade Key. After the Jade Key is discovered, the movie barely stops moving long enough to let the audience know what the riddle is that leads to the Crystal Key, before revealing the answer and moving on. You'd be forgiven for not even being able to remember what the riddle was, since it gets basically ignored. The film also never clearly explains how the solution was found. So let's do that, shall we?
After Art3mis locates the Jade Key inside a particular classic horror movie which I'm not going to spoil, we get a brief scene in the real world of Samantha and Wade trying to solve the new clue she was given. The clue reveals that if you take the "Number Ultimate" and divide it by the "Number Magic," you will find the location of the "Fortress Tragic." However, before the pair are able to begin trying to decode the riddle in any meaningful way, IOI breaks into the building and all hell breaks loose. Samantha helps Wade escape before she gets captured by IOI.
In the next scene, we see Wade meet up with the real world people behind his friends Aech, Diato, and Sho where he's informed that IOI has already solved the riddle. Helen (Aech) tells him that IOI knew they needed to look in sector 14, and eventually discovered the key's location by searching every fortress in the sector. So we know the answer is 14, but here's how you come up with it.
Let's start with the "Number Ultimate." This may be the easiest part of the riddle to decipher, as the ultimate number is pretty famous, even if you're not directly familiar with the source material that created it. That would be The Hitchhiker's Guide to the Galaxy. The work by Douglas Adams began life as a radio production, though likely became the most famous in book form, published in 1979. It tells the story of Arthur Dent, one of the last human beings in the universe after the planet is destroyed to make way for a hyperspace bypass. Dent learns that the Earth was actually a massive computer built to help discover the ultimate question to life, the universe, and everything. The reason the question was being investigated was that the answer had already been learned. The answer to life, the universe, and everything, it turned out, was 42. Check out this clip from the 2005 movie for the full explanation.
Now, the Number Magic is probably a bit harder for many people, although, if you've read the Ready Player One book you know the answer already, as this number is the one part of the hunt for the egg that actually survives the book and makes it into the movie, though the way it's used in the book is quite different. The other people who will know the answer are those who grew up watching Schoolhouse Rock on TV, because they know that "Three is a Magic Number."
So that's the solution to locating the Crystal Key. It's not surprising that IOI solved it so quickly, it's not that complicated, but it is fun, which makes it unfortunate that Ready Player One couldn't find the time to have the explanation make it to the screen.