What number should replace the question mark

What number should replace the question mark

What number should replace the "?" in the circle ?
Note : There aren't any errors or typos in the above image.

This number puzzle looks very easy but the moment you think that you have found the solution, you get to know that you are wrong, as soon as you apply your logic at the bottom of the puzzle.

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Answer :
The answer is 12.

Explanation:
Instead of finding the difference in the numbers,
try adding the digits of the numbers.
For eg. 7+2(72) + 9+9(99) is 9 + 18 = 27.
So the answer comes to,
2+1(21) + 3+6(36) = 3+9 = 12.