JEE Main & Advanced Mathematics Sets Question Bank Mock Test - Sets

  • question_answer
    In a town of 10,000 families, it was found that 40% families buy newspaper A, 20% buy newspaper B and 10% buy newspaper C. Also, 5% families buy newspapers A and B 3% buy newspapers B and C and 4% buy newspapers A and C. if 2% families buy all the three newspapers, then number of families which buy newspaper A only is _________.

    A) 3300

    B) 3200

    C) 3000

    D) 3400

    Correct Answer: A

    Solution :

    [3300]
    \[n(A)=40\,percent\,\,of\,\,10,000=4,000\]
    \[n(B)=20\,percent\,\,of\,\,10,000=2,000\]
    \[n(C)=10\,percent\,\,of\,\,10,000=1,000\]
    \[n(A\cap B)=5\,percent\,\,of\,\,10,000=500\]
    \[n(B\cap C)=3\,percent\,\,of\,\,10,000=300\]
    \[n(C\cap A)=4\,percent\,\,of\,\,10,000=400\]
    \[n(A\cap B\cap C)=2\,percent\,\,of\,\,10,000=200\]
    Number of families which buy newspaper A only
    \[=n(A\cap {{B}^{c}}\cap {{C}^{c}})\]
    \[=n[(A\cap {{B}^{c}}\cup {{C}^{c}})]\]
    \[=[n(A)-n[A\cap (B\cup C)]\]
    \[=[n(A)-n[(A\cap B)\cup (A\cap C)]\]
    \[=[n(A)-[n(A\cap B)+n(A\cap C)-n(A\cap B\cap C)]\]
    \[=4000-[500+400-200]\]
    \[=4000-700=3300\]
     


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