3. (7 points) There are 6 blue, 7 red, and 9 white bulbs. In how many ways can they be arranged into a garland (using all the bulbs) so that no two white bulbs are adjacent?
Answer: 3435432

Solution: First, arrange all the blue and red bulbs in $C_{13}^{6}$ ways. In the gaps between them and at the ends, choose 9 positions and insert the white bulbs. There are $C_{14}^{9}$ ways to do this. In total, there are $C_{13}^{6} \cdot C_{14}^{9}$ ways to compose the garland from the available bulbs.