1. **State the problem:** Ana has 72 books and each shelf can hold 7 books. She estimates needing 10 shelves. We need to check if this estimate is reasonable. 2. **Formula used:** To find the number of shelves needed, divide the total books by the capacity per shelf: $$\text{Number of shelves} = \frac{\text{Total books}}{\text{Books per shelf}}$$ 3. **Calculate:** $$\frac{72}{7} = 10.2857...$$ 4. **Interpretation:** Since you cannot have a fraction of a shelf, Ana needs to round up to the next whole number. 5. **Conclusion:** Ana needs 11 shelves, not 10, to hold all 72 books. Therefore, Ana's estimate of 10 shelves is not reasonable because it is too low. She actually needs 11 shelves.