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

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

If eleven plus two equals one, what does nine plus five equal?

John was visiting his friend Jacob. He found out that his friend's wife had just killed a burglar in self-defense.
John asks Jacob about what happened and he told that "His wife was watching television when suddenly the bell rang. She thought that it is her husband Jacob but she found the burglar who attacked her instantly and she got so frightened that she killed the burglar immediately with the knife.
John asked the police to arrest his friend's wife. Why?

Can you replace the question mark with the correct number?

John is the son of Jacob.
So,
Jacob is the_____of John's father?

A man who was outside in the rain without an umbrella or hat didn’t get a single hair on his head wet. Why?

Can you divide the below-given shape into four equal and identical pieces?

I possess a head

I flaunt a tail

But I don't have mouth or legs

Can you find out who I am?

It is a 5 letter word if you take away first letter it is something you get from sun, if you remove second letter you will get something to eat, if you remove third letter you get a word you use in pointing at and if you remove the fourth letter you get something to drink. What is it?