Good Riddles

Question: What single line can you add to the following statement to make it true?

5+5+5=550

Question: You are a prisoner. The only way to escape the prison is to walk down a very long hallway that takes 2 hours to get through. Guards come out of a door halfway down the hall every hour and take any prisoners walking out back to the prison and escort any visitors out of the prison. The guards cannot tell the difference between prisoners and visitors.

How do you escape?

Question: I used to fill rooms but now I'm a tower,
I can stay put or I can travel far.

What am I?

Question: It brings light to a dark dark world,
In a current it travels and through tunnels it's hurled.
Touch it and it'll kill you.

What is it?

Question: You fill up a bathtub with water and you have a teaspoon, tablespoon, and fourth cup. What is the fastest way to empty the bathtub?