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