Math Riddles

Question: What temperature is twice as hot as 0 degrees Celsius?

Question: You have two books. One of them is upside-down and the other is rotated so the top of the book is facing you.

What is the sum of the first page in each book?

Question: If you have two twins, three triplets and four quadruplets; how many people do you have?

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?