Replace the question mark with the correct number in the below-given picture?

Can you place six X (crosses) in below board without making three in a row in any way ?

10 people came into a hotel with 9 rooms and each guest wanted his own room. The bellboy solved this problem.
He asked the tenth guest to wait for a little with the first guest in room number 1. So in the first room, there were two people. The bellboy took the third guest to room number 2, the fourth to number 3, ..., and the ninth guest to room number 8. Then he returned to room number 1 and took the tenth guest to room number 9, still vacant.
How can everybody have his own room?

How many people must be gathered together in a room, before you can be certain that there is a greater than 50/50 chance that at least two of them have the same birthday?

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

If you drop me I’m sure to crack, but give me a smile and I’ll always smile back. What am I?

An old Woman left INR 33,333 to be divided equally among two fathers and two sons and each was to receive INR 11,111.
How was this possible?

Your mother’s brother’s only brother-in-law is asleep on your couch. Who is asleep on your couch?

Find the next number in the series

12, 20, 40, 72, 116, 172 ?

One day, I thought of ways that can be used for creating a palindrome. So I decided that I will turn into a larger number by adding the reversed digits to the original number and keep doing it till I finally obtained a palindrome.

I am not sure if this process will always result in a palindrome eventually but I was able to produce a four-digit palindrome. Can you guess my starting number?