In the "sum" below one of the digits is missing, marked by a question mark. Clearly this is not an addition, subtraction or any normal sum. So what is going on?
57944
81039
87025
-------
75?36
What digit has been replaced by the question mark?
In the "sum" each digit below the line is the average of the three digits above it above the line. 9 plus 0 plus 0 equals 9, giving an average of 3.