Mentor: Suppose we had to figure out how many books are in a bookcase. We need the answer right away, so there is not time to count each book. What do we do?

Student: I know! We make an estimate.

Student 2: Yes. We guess how many books are in the bookcase. One way to do it quickly would be to count how many books are on one shelf, and then multiply that number by the number of shelves. For example, if there were 20 books on one shelf, and 4 shelves in the bookcase, that would be 20 x 4, or 80. Our estimate would be that there are about 80 books.

Mentor: Very good. If you did it that way you probably would not get the exact answer. But you would be close enough, and you would have found the answer in a short time.

Student: What if there were two bookcases and we wanted to know which one held more books?

Mentor: Then we would be making a comparison estimate, where we would estimate how many books are on each bookcase and say which one had more.

Student 2: So the number of books on one bookcase will be greater than, less than, or almost the same as,the number of books on the other.

Mentor: Correct. It isn't necessary to have the exact number for an estimate, just an approximate idea.