Look closely at the alphabets on the elevator door. Notice that it didn't have letter E, I, O, and U.
- E is the 5th letter in the alphabet.
- I is the 9th letter in the alphabet.
- O is the 15th letter in the alphabet.
- U is the 21st letter in the alphabet.
Tap the keypad on the left and key in: 591521.
Elevator opens. Welcome to level 51.