A charity organization is collecting donations of two types: large bags of clothing and small boxes of books. Each large bag of clothing is valued at $$120$ , and each small box of books is valued at $$80$ . The total value of donations collected on a particular day was $$600$ . The equation $120 C + 80 B = 600$ represents the number of large bags of clothing, $C$ , and the number of small boxes of books, $B$ , collected. If the charity collected 3 large bags of clothing, how many small boxes of books did they collect?