I am white when I am dirty, and black when I am clean. What am I?

How many points are there on the globe where, by walking one mile south, then one mile east and then one mile north, you would reach the place where you started?

Can you solve the below rebus?

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

You have an empty wine bottle with a cork that has been secured at the top in a normal way. There is a metal ring inside the bottle that is suspended by a string.

How can you make the metal ring drop to the bottom if you are not allowed to touch anything - not the bottle, not the cork, not the thread and not the ring?

How can a man go eight days without sleep?

Decode below three cipher riddles:

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

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?

You’ll find me on Mercury, Earth, Mars, Jupiter, Saturn, and Uranus. But never Neptune, or Venus. What am I?

A chess tournament is taking place on knock-out terms (the one who loses the match is out of the game).
(a) If 10 matches are played in total, how many players participated?
(b) If 20 players took part in the tournament, how many matches were played?