Which number should replace the question mark?

I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

A chess tournament is taking place on knock-out terms (the one who loses the match is out of the game).
(a) If 10 matches are played in total, how many players participated?
(b) If 20 players took part in the tournament, how many matches were played?

What 5-letter word typed in all capital letters can be read the same upside down?

Here is a conversation between a couple who both are software professionals.

Husband: Hey! What is your date of birth?
Wife Replied Angrily: It is 591062400
Husband after doing some calculations replied, "Got It".

What is the DOB of the wife?

You see a boat filled with people, yet there isn’t a single person on board. How is that possible?

Find The Next Number in the series:

12 13 15 17 111 113 117 119 123 ?

Jenifer is learning to drive her car. She went down a one way lane and in the wrong direction. But she do not break any law.

How come ?

In a school, there are four subjects. Seventy percent of students study English, seventy five percent of students study Science, eighty five percent of students study Mathematics and eighty percent of students study Spanish.

Can you calculate the percentage of students that study all four subjects?

What time is it when an elephant sits on a fence?