A4. Books are arranged on three shelves in a wardrobe. On the top shelf, there are 30 books, which represents one third of all the books. On the middle shelf, there are 10 more books than on the bottom shelf. How many books are on the bottom shelf?
(A) 25
(B) 30
(C) 35
(D) 40
(E) 60
A4. The 30 books represent a third of all the books, so there are 90 books in total. The middle and lower shelves together have 60 books. Let's say there are $x$ books on the lower shelf. Then there are $x+10$ books on the middle shelf. We can write the equation $x+x+10=60$, which has the solution $x=25$. There are 25 books on the lower shelf.