If 21x = 79x, what is the value of x?

Solve the following series:
AZ, GT, MN, __, YB

There are three Athletes (John, Tarun and Harish) and their individual Coaches (Jacob, Meenaxi and Priyanka) standing on the shore.
No Coach trusts their Athlete to be near any other Coach unless they are also with them.
There is a boat that can hold a maximum of two persons.
How can the six people get across the river?

A man was found murdered on Sunday morning. His wife immediately called the police. The police questioned the wife and staff and got these alibis: The Wife said she was sleeping. The Cook was cooking breakfast. The Gardener was picking vegetables. The Maid was getting the mail. The Butler was cleaning the closet. The police instantly arrested the murderer. Who did it and how did they know?

Eight of us go fourth, not back, to protect our king from an attack. What are we?

A man was just doing his job when his suit was torn. Why did he die three minutes later?

The average age of 10 members of a committee is the same as it was 4 years ago, because an old member has been replaced by a young member. Find how much younger is the new member ?

You can take four of the five letters out of this word, but the pronunciation never changes. What is the word?

What happened when the wheel was invented?

A word I know,
Six letters contain,
Subtract just one,
And twelve is what remains.