9 out of 10 people answer this question wrong

With your calculation and reasoning skills, please give the correct answer to the question in the picture. You may not know, this seemingly simple problem makes many people answer incorrectly because of haste and lack of concentration. So please stay calm to conquer this interesting question.

The answer seems to be hidden behind the hints, just pay attention and you will find it.

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

Trung Temple