Question

There are three copies each of 4 different books. In how many ways can they be arranged in a shelf?

Answer 1

Total number of books = 12
∴ Required number of arrangements = Arrangements of 12 things of which each of the 4 different books has three copies = $\frac{12!}{3!3!3!3!}$ = $\frac{12!}{(3!{)}^4}$