Math problem: There are a number of books in the library. After lending 40%, I bought 360 books. Now the ratio of books to original works is 3: 4. How many books are there?
Arithmetic solution:
After lending 40%, the remaining 60% becomes 3/4=75% after buying 360 copies.
Then 360 books are 75%-60% of the original books =15%.
So the original book =360÷[ 3/4-( 1-40%) ]=2400.