Answer: The sum of the numbers in the three corners of each large square is the ordinal number of the letter in the remaining corner calculated according to the English alphabet. We have: 6 + 4 + 4 = 14. Looking on the English alphabet we get the letter N. Similarly, 1 + 2 + 14 = 17. In conclusion, the letter that needs to be filled in with the question mark is the 17th letter in the English alphabet. That's the letter Q.
