Replace the question mark with the number below given a simple math problem.

A girl was sitting in her hotel room when she heard a knock on the door. She opened the door and found that a man was standing outside.
The man said, "Oh! I am really sorry, I thought this was my room."
He then walked through the corridor to the elevator. The girl did not know the man. She closed her door and called security asking them to apprehend the man.
What made her suspicious of that man? He might have been genuinely mistaken.

I have branches, but no fruit, trunk or leaves. What am I?

Why are all black people so quick ?

Can you make a six-letter word using the letters N, A and B ?

Can you decipher the two rows to find the hidden word?

What can run but never walk, have a mouth that never speaks, have a head that never weeps, and have a bed but never sleeps?

Can you write down eight eights so that they add up to one thousand?

A B C D E F G H

These are the letters given to you. Now you have to find out the letter that comes two to the right of the letter which is immediately to the left of the letter that comes three to the right of the letter that comes midway between the letter two to the left of the letter C and the letter immediately to the right of the letter F.

Spot 5 differences in two Christmas tree below: