In a survey of college students it was found that `40%` use their own books, `50%` use library books, `30%` use borrowed books, `20%` use both their own books and library books. `15%` use their own books and borrowed books, `10%` use library books and borrowed books, and `4%` use their own books, library books and borrowed books. Calculate the percentage of students who do not use a book at all.

Answer

`21%`