Find the missing letter in the last circle logically in the given below picture.

The host of a game show offers the guest a choice of three doors. Behind one is an expensive car, but behind the other two are goats.
After you have chosen one door, he reveals one of the other two doors behind which is a goat (he wouldn't reveal a car).

Now he gives you the chance to switch to the other unrevealed door or stay at your initial choice. You will then get what is behind that door.

You cannot hear the goats from behind the doors, or in any way know which door has the prize.

Should you stay, or switch, or doesn't it matter?

What is next number in the pattern below:

131 517 192 123 ?

A man says, "Brothers and sisters, have I none, but that man's father is my father's son." Who is that man?

Can you count the number of squares in the figure below?

Different lights make me strange by changing me into different sizes. What am I?

In a football game, Christina defeated Teresa by 2 - 0 but no man from either side scored. How can this be possible?

Before Mount Everest was discovered, what was the highest mountain in the world?

What number comes next in this sequence:
7 8 5 5 3 4 4 _

Christina, Allison and Lena are 3 daughters of John a well-known Mathematician, When I asked John the age of their daughters. He replied "The current age of her daughters is prime. Also, the difference between their ages is also prime."

How old are the daughters?