The centre of the third figure is empty. What digit should fit inside?

Look at the below sequence of letters and tell us what the next letter should be:

O T T F F S S ?

In two decks of cards, what is the least amount of cards you must take to be *guaranteed* at least one four-of-a-kind?

What will be the best approach to finding all the prime numbers less than 75 that leave an odd reminder when we divide them with 5?

What is the Surname of Barbie Doll?

I am a type of word that is spelt incorrectly or used differently, on purpose for humour. What am I?
Hint 1: You can find me in a joke.
Hint 2: I am a play on words.

A red house is made out of red bricks. A yellow house is made out of yellow bricks. And, a blue house is made out of blue bricks. So, what is a greenhouse made of?

How can you drop a raw egg from a height onto a concrete floor without cracking it?

On a charity event in Russia, Murray beat Djokovic by winning six games and losing three games.
Note: There is a total of 5 service breaks(one who serve lost the game).
Who served first, Djokovic or Murray?

Railroad Crossing, look out for the cars. Can you spell that, without any R's ?