1. **State the problem:** We need to find how many books are in a set if the total cost is 72 and each book costs 4.50. 2. **Formula used:** To find the number of books, use the formula: $$\text{Number of books} = \frac{\text{Total cost}}{\text{Cost per book}}$$ 3. **Apply the values:** $$\text{Number of books} = \frac{72}{4.50}$$ 4. **Simplify the division:** $$\frac{72}{4.50} = \frac{72}{\cancel{4.50}} \times \frac{\cancel{1}}{1} = 16$$ 5. **Interpretation:** There are 16 books in the set. **Final answer:** $$\boxed{16}$$