Q.
The number of ways can 10 letters be placed in 10 marked envelopes, so that no letter is in the right envelope is
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a
10!{1−11!+12!−13!+......+110!}
b
10!{1+11!−12!+13!−......−110!}
c
{1+11!−12!+13!−......−110!}
d
9! {1+11!−12!+13!−......−110!}
answer is A.
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Detailed Solution
The number of ways can 10 letters be placed in 10 marked envelopes, so that no letter is in the right envelope is 10![1−11!+12!−13!+−−−−+110!] Derangement formula n!1-11!+12!-13!+........+(-1)n1n!
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