Math Riddles

Question: There are several books on a bookshelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?

Question: You have found a mutant algae that doubles in size every hour. It takes 18 hours for one algae plant to take up half of a certain lake.

How long would it take for two of these plants to take up half of the same lake?

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?