Final answer:
The shelf can hold a maximum of 10 books, each 10 cm thick.
Step-by-step explanation:
To find out how many books, each 10 cm thick, the shelf can hold, we need to calculate the number of books that can fit within the available shelf space. The thickness of the books is given as 5 cm. Therefore, the available space for books on the shelf is 21 books * 5 cm = 105 cm.
Now, we divide the available space by the thickness of each book: 105 cm / 10 cm = 10.5 books.
Since we cannot have half a book, the shelf can hold a maximum of 10 books, each 10 cm thick.