At first glance, this little brain teaser looks very simple. You only see four boxes and three letters: A, C, and G. One box is empty, and the question asks: What letter is missing?
Many people immediately try to read it like a normal alphabet sequence. They see A, then C, and they think the missing letter might be B. But that does not fit with the letter G below.
The trick is to look at the letters as numbers in the alphabet.
A = 1
C = 3
G = 7
Now look at the pattern:
From A to C, we move forward 2 letters.
A → B → C
So if the same idea continues downward or across the grid, the numbers are growing in a pattern:
1, 3, 7, ?
The difference from 1 to 3 is +2.
The difference from 3 to 7 is +4.
So the next difference should be +8.
That means:
7 + 8 = 15
The 15th letter of the alphabet is O.
So the missing letter is:
O
The full pattern is:
A = 1
C = 3
G = 7
O = 15
Each step doubles the difference:
+2, +4, +8
That is why the answer is O, not B, D, or I.