JEE Main & Advanced Mathematics Question Bank Critical Thinking

  • question_answer
    The probability that a leap year selected at random contains either 53 Sundays or 53 Mondays, is          [Roorkee 1999]

    A)                 \[\frac{2}{7}\]       

    B)                 \[\frac{4}{7}\]

    C)                 \[\frac{3}{7}\]       

    D)                 \[\frac{1}{7}\]

    Correct Answer: C

    Solution :

               A leap year consists of 366 days comprising of 52 weeks and 2 days. There are 7 possibilities for these 2 extra days viz.            (i) Sunday, Monday,            (ii) Monday, Tuesday,            (iii) Tuesday, Wednesday, (iv) Wednesday, Thursday,            (v) Thursday, Friday,           (vi) Friday, Saturday and            (vii) Saturday, Sunday.            Let us consider two events :            \[A:\] the leap year contains 53 Sundays            \[B:\] the leap year contains 53 Mondays.            Then we have \[P(A)=\frac{2}{7},\,\,P(B)=\frac{2}{7},\,\,P(A\cap B)=\frac{1}{7}\]            \[\therefore \] Required probability \[=P(A\cup B)\]                 \[=P(A)+P(B)-P(A\cap B)=\frac{2}{7}+\frac{2}{7}-\frac{1}{7}=\frac{3}{7}.\]


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