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In a class of 10 students roll numbers a...

In a class of 10 students roll numbers are written on their respective chairs. Find the ways in which exactly 3 students do not sit on their chairs .

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Ways of derangement `=""^(10)C_(3).3![1-(1)/(1!)+(1)/(2!)-(1)/(3!)]`
`=(10xx9xx8)/(3xx2)xx3xx2xx1[(1)/(2)-(1)/(6)]=240`
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