Can you replace the question mark with the correct number?

Replace the question mark with the correct number below.

What brings you down but never up?

A seven-year-old kid challenged his classmates that he can make the number one disappear by adding something to it.

How can he do that?

A mother is 21 years older than her child. In exactly 6 years from now, the mother will be exactly 5 times as old as the child.
Where's the father?

I am soft and cuddly that soothe your heart. But, If you pick my last name I am going to tear you all apart. Who am I?

You walk into a room that contains a match, a kerosene lamp, a candle and a fireplace. What would you light first?

You measure my life in hours
I serve you by expiring
I vanish faster when I am thin
and slower when I am thick
The wind is my energy
What am I?

If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

In the Chess Board picture below white army is arranged. You need to add a black army on the board such that no piece is under any threat.
Note: Army comprised of 1 king, 1 queen, 2 rooks, 2 bishops, 2 knights, and 8 pawns.