You're running late to your first day on the job and after arriving at the meeting location, you see the orientation group just leaving without you through a door with a keypad.
Panicked, you confirm the door re-locked, and then pound on the door with no answer. In a last ditch effort, you ask the only person around, a janitor, if he knows the combination.
You ask him to repeat himself since he's mumbling slightly.
He shouts at you, enunciating slowly and clearly, "24 equals!". Then he walks away looking angry.
After saying your thanks, you try it but immediately realize the combination isn't long enough. To submit a combination, 4 of either the numbers, X, or + need to be entered followed by the equal sign.
What do you do?
NOTE: Guesses are welcome, I can say if the combination works so you can put yourself in the story. There is no limit to the number of guesses.
NOTE 2: Random guessing isn't necessary (or how could this be considered a puzzle?)
You notice the janitor doesn't seem like a guy who likes riddles or math.