I need a clearer understanding of how to solve: To make room for a new inventory of books, Gordon's local bookstore is offering 30% off any book that is over $22.00 and 20% off any book under $20.00.  He buys several books with the following prices; $25.00, $18.00, $21.00, $35.00, $12.00 and $10.00.  How much will he spend on books?