Math Riddles

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

5+5+5=550

Question: There are five hen and rooster pairs. Each pair has one baby every month.

How many chickens will there be in one year?

Question: You find me behind the stars; or in a sixth, seventh, or third.
It takes something round, a computer, and me to make pie.
I am bigger than anything you can think of.

What am I?

Question: When is 1500 plus 20 and 1600 minus 40 the same thing?

Question: What is the difference between a dollar and a half and thirty five-cents?