Curiosity Corner sells books and various other reading$-$related products. One of the store's most popular reading$-$related products is a book pillow for hard cover and soft cover books. The pillows each sell for $400.$ Originally the pillows were handmade by a local artisan. The store's owner has been impressed with the demand for the pillow and has recently begun a small manufacturing company to produce and distribute the pillows to other stores. Estimated sales for the fourth quarter $($in units$)$ are as follows:

$\backslash$table$[[$October$,6,500],[$November$,7,200],[$December$,9,{600}_{?}$