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In a survey of $100$ students, it was found that $50$ used the college library, $40$ had their own library and $30$ borrowed books of these, $20$ used both the college library and their own, $15$ used their own library and borrowed books and $10$ used the college library and borrowed books. How many students used all the three sources of books ?

- $30$
- $28$
- $20$
- $25$