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

There are 20 people in an empty, square room. Each person has full sight of the entire room and everyone in it without turning his head or body, or moving in any way (other than the eyes). Where can you place an apple so that all but one person can see it?

What is the difference between a jeweller and a jailer?

If you're 8 feet away from a door and with each move you advance half the distance to the door. How many moves will it take to reach the door?

I’m orange, I wear a green hat and I sound like a parrot. What am I?

A man is lying dead in a field where no one is around. His head is split open and his legs are disfigured. Near to him, there is an unopened package. No living organism can be found anywhere at the crime scene.

How did he die?

What Adam and Eve do not have but the rest of the people have?

Name one eight-letter word with 'kst' in the middle, 'in' the beginning, 'and' at the end.?

What is the largest amount of money that you can possess in change and still if someone asks you for a dollar, you won't be able to give?

There are 2 sand hourglasses.

The small one can measure 5 hours and the large one can measure 7 hours.

How can we measure 16 hours with 2 sand hourglasses running together ?