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The number of ways of arranging 10 books on a shelf such that 2 particular books are always together is
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We can mark the particular books as S1 & S2.

And the other books as B1, B2, B3, B4, B5, B6, B7, B8.

The particular books are always together.

So, we can arrange this (8 + 1) = 9 books in 9! way in the shelf AND the particular books can itself arranged in 2! ways.

So, Total ways = 9! * 2!

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