Which number should replace the question mark?

Can you think of any three-dimensional shape that comprises of exactly two surfaces?

Solve the equation by replacing the alphabet with numbers.

MOM + MOM + NO = BOOK=

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see?

Who always enjoys poor health?

In a chess board, the queen piece can move horizontally, vertically and diagonally freely. The picture represents the same.Can you place 8 queens on the board in a manner that none of the queens can attack each other?

You and your two friends are working in a multinational company. How can you three find out the average salary of you all without disclosing your own salary to the other two?

Can you find out which triangle will have a bigger area among the following?
1. A triangle with sides 300, 400 and 500
2. A triangle with sides 300, 400 and 700

When it's bad luck to meet a black cat?

What is in seasons, seconds, centuries and minutes but not in decades, years or days?