why number 8549176320 is one of a kind?

What 4-letter word can be written forward, backward or upside down, and can still be read from left to right?

A woman lives in a skyscraper thirty-six floors high and is served by several elevators which stop at each floor going up and down. Each morning she leaves her apartment and goes to one of the elevators. Whichever one she takes is three times more likely to be going up than down. Why?

Take away my first letter, then take away my second letter. Then take away the rest of my letters, yet I remain the same. What am I?

Place a mathematical symbol between the numerals 5 and 9 in such a way that the resulting number is greater than 5 but smaller than 9.

Forwards I am heavy, backward I am not. What am I?

Two boys wish to cross a river. The only way to get to the other side is by boat, but that boat can only take one boy at a time. The boat cannot return on its own, there are no ropes or similar tricks, yet both boys manage to cross using the boat.

How?

What type of tree can I carry in my hand?

How many times do a clock's hands overlap in a day ?

Can you replace the question mark with the correct letter?

6 + 6 + 6 = N
7 + 7 + 7 = E
8 + 8 + 8 = R
0 + 0 + 0 = ?