This Application Problem is a part of the Lesson 7, Unit 2, Grade 2. Luigi has 9 more books than Mario. Luigi has 52 books. How many books does Mario have? This compare with smaller unknown problem has the complexity that we subtract to find the number of books Mario has, though there is no action of taking away, and the word more in the first sentence might suggest addition to students. More and more than are often mistakenly taught as key words signaling either to add or subtract.