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Need help with (c) part ii (1 Viewer)

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kimtuluu

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The solution provided is
1625629269651.png
Please help explaining why we have to subtract 1 when we want to select at least 1 book. Many thanks in advance
 
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CM_Tutor

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Because the includes picking any of the copies of a book, or not picking it. So, it includes the possibility of not picking a copy of any of the books. To pick at least 1 book, the one option of picking no books needs to be removed.
 
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kimtuluu

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CM_Tutor said:
Because the includes picking any of the copies of a book, or not picking it. So, it includes the possibility of not picking a copy of any of the books. To pick at least 1 book, the one option of picking no books needs to be removed.
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Got it. Many thanks.
 
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