Stuck trying to solve this question that popped up the other day. Spent the whole of the 15 minute test trying to find an answer to it. 
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2 Answers
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The answer is
$2$
because
in each box, label the numbers like this:
A B
C DThen $B * C = 10A + D$.
i.e. $3 * 6 = 18$, $7 * 8 = 56$, and $9 * 3 = 27$. So in the final box, we need a $2$ to complete the equation $5 * 4 = 20$.
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I hate this kind of puzzles, since the solution always feels kind of arbitrary to me. Just for the sake of it I suggest an alternative solution:
1
Reason:
The four boxes form a table with four rows and four columns. In each column, label the numbers like this: A B C D Then (A+B+C+D) mod 3 = 0
Now that I think about it, using the same argument, even
7
would be a solution. Depending on your creativity, it appears to me that any given number might be a solution to this problem.
