1. **Stating the problem:** We need to find the minimum thickness of a book that has a front and back cover each 0.9 mm thick (to 1 decimal place) and 400 pages each 0.13 mm thick (to 2 decimal places). 2. **Calculate thickness of covers:** Since both the front and back covers are 0.9 mm thick each, total cover thickness is: $$0.9 + 0.9 = 1.8 \text{ mm}$$ 3. **Calculate thickness of pages:** There are 400 pages, each 0.13 mm thick, so total page thickness is: $$400 \times 0.13 = 52.0 \text{ mm}$$ 4. **Calculate minimum total thickness:** Adding covers and pages: $$1.8 + 52.0 = 53.8 \text{ mm}$$ **Final answer:** The minimum thickness of the book is **53.8 mm**.