What is the fewest number of coins that would be required in order to make sure each and every coin touches exactly three other coins?

There are 20 people in an empty, square room. Each person has full sight of the entire room and everyone in it without turning his head or body, or moving in any way (other than the eyes). Where can you place an apple so that all but one person can see it?

Decode below three cipher riddles:

26 L of the A
7 D of the W
1000 Y in a M

Solve the equation by replacing the alphabet with numbers.

MOM + MOM + NO = BOOK=

What's the only room from which no one can enter or leave

What is the word or phrase?

The answer I give is yes, but what I mean is no. What was the question?

There are two words one resembles the "state of rest" and the other is related to "writing/reading material".

Can you name these two words which also sound similar?

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

A landlord calls both of his sons and tells them that their horses will now decide who will transfer the inheritance. He tells them to race along the land till the end and the one whose horse will be slower will win and be the heir to all the property.

Both of them keep wandering for days but to no result. Then they ask a wise man regarding it. The man advises them on the matter after which they jump on the horses and race as fast as they can till the end. Why did they do it?

What did the wise man say?