Question:
There are three copies each of 4 different books. In how many ways can they be arranged in a shelf?
Solution:
Total number of books = 12
$\therefore$ Required number of arrangements $=$ Arrangements of 12 things of which each of the 4 different books has three copies $=\frac{12 !}{3 ! 3 ! 3 ! 3 !}=\frac{12 !}{(3 !)^{4}}$
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