Hello! I was just curious of why this is the correct answer? Thank you!

The Abominable Snowman has sold a million copies of his new book of lifestyle advice. Unfortunately, due to a production glitch, 40% of them are missing the last chapter. Used book dealers cannot tell the difference u ntil after they purchase the book. The used book market is highly competitive, with many dealers. The current owners of good copies of the book will sell their books if and only if they get at least $15, and the owners of bad copies will sell theirs if and only ifthey get at least $2. A good copyis worth $20 to a dealer, and a bad one is worth $5. What is the average value to a dealer of all the books? 3120 $9.80 $12.50 $11