Using your calculation and reasoning skills, complete the task of finding the answer to this puzzle.

You may not know, this seemingly simple question has "knocked out" many people because of haste and lack of concentration.

Be calm and conquer this interesting question.

The answer seems to be hidden behind the hints. Just look closely and you'll find it.

Quickly register yourself in the top smartest people by leaving your answer in the comment box below.

Trung Temple