Which number should replace the question mark?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

Assume the given figure to be a delicious doughnut. Yes, now you can concentrate more on the puzzle. So you have this delicious doughnut in your refrigerator when your friends come knocking at the door. There are eight of them. Now you have to make three cuts in this doughnut so that each one of you nine people can enjoy a piece of it. Neither you nor your friends would mind the size of their piece as long as they are getting it. How will you do it?

You can see three almost identical images below. Can you spot the odd one out from them?

A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?

Using mathematical symbols like +, -, and x in order to make the equations correct. Replace # with the above-given symbols.

9 # 8 # 7 # 6 # 5 # 4 = 91

It spends most of its day eating white, but when it’s quick enough, it gets to eat fruit and sometimes some blue things. It’s in a dark room, where the walls are blue, it runs from a ghost that roams the halls and haunts it all the time. What is it?

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

Complete the following series. It.s very logical:

1, 4, 5, 6, 7, 9, 11, __?

Prove that the below maths equation is true.

8 + 8 = 91