this one's fun

During a sale, a bookstore sold 1/2 of all its books in stock. On the following day, the bookstore sold 4,000 more books. Now, only 1/10 of the books in stock before the sale are remaining in the store. How many books were in stock before the sale?

source:
SSAT & ISEE 2007 Edition Kaplan
p. 161

C. came pretty close to doing this one on his own.

Of course, close doesn't cut it on a standardized test.

I realize that.