In how many ways can 4 red,
Question:

In how many ways can 4 red, 3 yellow and 2 green discs be arranged in a row if the discs of the same colour are indistinguishable?

Solution:

Number of red discs = 4

Number of yellow discs = 3

Number of green discs = 2

Total number of discs = 9

Total number of arrangements = Number of arrangements of 9 things of which 4 are similar to the first kind, 3 are similar to the second kind and 2 are similar to the third kind

$=\frac{9 !}{4 ! 3 ! 2 !}=1260$