A student has one book each on ten subjects. If the Hindi book, Mathematics book and Science book are to be always kept together, how many arrangements of books are possible?

Treat the three specific books as a single combined unit, which means there are now the 7 remaining individual books plus this 1 unit, giving 8 entities to arrange. Using the fundamental principle of counting, these 8 entities can be arranged in 8! different ways. The three books within the combined unit can also be arranged among themselves in 3! ways, so multiplying these together gives the total arrangements as 8! 3!.

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