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

A pack of cards has 40 cards. You are blindfolded. Out of 40, 25 cards are facing down while 15 are facing up. You have been asked to divide this pack of cards into two decks - so that each deck contains an equal number of face-up cards. Remember, you are blindfolded.

How will you do it?

I have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?

Prove that the below maths equation is true.

8 + 8 = 91

Accidentally, two trains are running in the opposite direction and enter a tunnel that is 200 miles long. A supersonic bird that has fled the lab and taken shelter in the tunnel starts flying from one train towards the other at a speed of 1000 mph. As soon as it reaches the second train, he starts flying back to avoid collision and meets the first train again at the other end. The bird keeps flying to and fro till the trains collide with each other.

What is the total distance that the supersonic bird has traveled till the trains collided?

What seven-letter word has hundreds of letters in it?

I am a seven-letter word but if you remove four of my letters I will have one left. What word am I?

0 0 0 = 6
1 1 1 = 6
2 2 2 = 6
3 3 3 = 6
4 4 4 = 6
5 5 5 = 6
6 6 6 = 6
7 7 7 = 6
8 8 8 = 6
9 9 9 = 6

You can use any mathematical symbols in the space provided to make all above algebraic expressions true.

Below the four parts have been reorganised. The four partitions are exactly the same in both arrangements. Why is there a hole?

How can you leave a room with two legs and return with six legs?