Maths-
General
Easy

Question

7 people leave their bags outside temple and returning after worshiping the deity picked one bag each at random. In how many ways at least one and at most three of them get their correct bags ?

  1. 7C3 .9 + 7C5 .44 + 7C1 .265  
  2. 7C6 .265 + 7C5 .9 + 7C7 .44  
  3. 7C5 .9 + 7C2 .44 + 7C1 .265  
  4. None of these  

The correct answer is: 7C3 .9 + 7C5 .44 + 7C1 .265


    No. of ways getting one correct
    = 7C1 6! open parentheses 1 minus fraction numerator 1 over denominator 1 factorial end fraction plus fraction numerator 1 over denominator 2 factorial end fraction minus fraction numerator 1 over denominator 3 factorial end fraction plus... plus fraction numerator 1 over denominator 6 factorial end fraction close parentheses
    = 7C1.265
    No. of ways getting two correct
    = 7C2. 5! open parentheses 1 minus fraction numerator 1 over denominator 1 factorial end fraction plus fraction numerator 1 over denominator 2 factorial end fraction minus fraction numerator 1 over denominator 3 factorial end fraction plus... negative fraction numerator 1 over denominator 5 factorial end fraction close parentheses
    = 7C2 . 44
    No. of ways getting three correct
    = 7C3.4! open parentheses 1 minus fraction numerator 1 over denominator 1 factorial end fraction plus fraction numerator 1 over denominator 2 factorial end fraction minus fraction numerator 1 over denominator 3 factorial end fraction plus fraction numerator 1 over denominator 4 factorial end fraction close parentheses
    = 7C3.9
    Required no. of ways= 7C3 . 9 + 7C2 . 44 + 7C1. 265

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