13. Grace is an egg farmer who sells eggs in two different-sized containers: a
dozen eggs for $5 or a 30-egg flat for $10.80. She is going to start selling an
18-ege carton as well and is wondering what price to charge. She wants to
set the unit price of the 18-egg carton between the unit prices for the other
two sizes. What price range can she choose from for the new size?