How can you make number one disappear by adding something to it?

An exterior architect is asked by a builder to plant seven trees in a manner that there are exactly six rows of trees in a straight line and each row has three trees in particular.

How will he do it?

I look at you, you look at me, I raise my right, you raise your left. What am I?

A woman is sitting in her hotel room and hears a knock at the door. She opens the door to see a man whom she’s never met before. He says, “I’m sorry, I have made a mistake, I thought this was my room.” He then goes down the corridor and into the elevator. The woman goes back into her room and calls security. What made the woman so suspicious of the man?

What is the word or phrase?

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

What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?

What has lots of eyes, but can’t see?

How many letters are in the alphabet?

Can you place six X (crosses) in below board without making three in a row in any way ?