This brain teaser shows four boxes with letters A, C, G, and one missing letter.
At first, it looks like normal alphabetical order, so many people guess B. But the trick is to convert letters into numbers:
A = 1
C = 3
G = 7
Now the pattern becomes: 1, 3, 7, ?
The differences increase: +2, then +4, so next is +8.
So: 7 + 8 = 15
The 15th letter is O.
Final answer: O