Final answer:
Daniela's book, with a perimeter of 28 inches and a width of 5 inches, has a length of 9 inches based on the perimeter formula for rectangles.
Step-by-step explanation:
The student Daniela measured the perimeter of her book to be 28 inches and knows that the width is 5 inches. To determine the length of the book, we can use the formula for the perimeter of a rectangle, which is P = 2l + 2w, where P is the perimeter, l is the length, and w is the width. Since we have the perimeter and the width, we can plug in these values and solve for the length. The equation becomes 28 = 2l + 2(5), simplifying to 28 = 2l + 10. Subtracting 10 from both sides yields 18 = 2l; dividing both sides by 2 gets us l = 9. Therefore, the length of Daniela's book is 9 inches.