Which graph represents a function?

The head librarian at the library of congress has asked her assistant for an interval estimate of the mean number of books checked out each day. the assistant provides the following interval estimate: from 740 to 920 books per day. what is an efficient, unbiased point estimate of the number of books checked out each day at the library of congress?

The sales totals at macy's food store have increased exponentially over the months. which of these best shows the sales in the first three months?

How is working backward similar to solving an equation?