Question:
Find the number of ways of selecting 9 balls from 6 red balls, 5 white balls and 5 blue balls if each selection consists of 3 balls of each colour.
Solution:
Required number of ways $={ }^{6} C_{3} \times{ }^{5} C_{3} \times{ }^{5} C_{3}=\frac{6 !}{3 ! 3 !} \times \frac{5 !}{3 ! 2 !} \times \frac{5 !}{3 ! 2 !}=2000$