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Class 11MathematicsPermutations and CombinationsPermutationsExpert
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When (7) letters are put into (7) addressed envelopes, how many ways have exactly (2) letters in correct envelopes?

Answer and explanation

Correct answer: (924)

Choose the correct letters in (^{7}C_2) ways and derange the remaining (5) letters in (44) ways. For exactly correct cases, all remaining objects must be wrong.

Related tags

PermutationsDerangementEnvelopesPermutations And CombinationsMathematicsClass 11 Mcq
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  5. 5Three distinct prizes are to be awarded to seven students. No student can receive more than one prize. How many ways are possible?
  6. 6If \(\frac{(n+5)!}{(n+3)!}=210\), what is the value of \(n\)?
  7. 7What is the value of \(\frac{21!}{19!\cdot2!}-\frac{18!}{16!\cdot2!}\)?
  8. 8If \(\frac{(n+1)!}{(n-1)!}-\frac{n!}{(n-2)!}=24\), what is the value of \(n\)?
  9. 9What is the value of \(\frac{22!}{20!\cdot 2!}-\frac{21!}{20!}\)?
  10. 10If (5) different books are arranged in a row, how many ways are possible?
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