Question:
There are 3 blue balls, 4 red balls and 5 green balls. In how many ways can they are arranged in a row?
Solution:
To find: no of ways in which the balls can be arranged in a row where some balls are of the same kind
Total number of balls $=3+4+5=12$
3 are of 1 kind, 4 are of another kind, 5 are of the third kind
Number of ways $=\frac{12 !}{3 ! 4 ! 5 !}=27720$
They can be arranged in 27720 ways