When you plot the data in a pie graph, it looks like that shown in the picture. A circle is formed when you draw a point and connect it from end to end forming a complete revolution. One revolution equals 360°. Therefore, each piece of pie for each type of book is a fraction of one revolution. The fraction can be determined by dividing the number for a specific type of book to the total number of books. Specifically, the fraction for the Self Help slice would be 90/375 which is equal to 0.24 or 24%. Then, 24% of 360 is 86.4. Therefore, the central angle formed by the Self Help slice is 86.4°.