The sum is unchanged, which is $$1400$$，$$1400\div \left( 3+1 \right)=350$$，$$1100-350=750$$． 
There are two bookshelves, A and B. Shelf A has $$1100$$ books, and shelf B has books $$300$$. We need to take  books from shelf A to shelf B, so that books in shelf B is three times more than self A. 

- $$750$$ 
- $$800$$ 
- $$850$$ 
- $$900$$ 