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    Question

    The average number of books on history, polity and

    geography in a library is 180, and the average number of books on history and geography is 150. If the number books on geography is 20% less than the number of books on polity, then what is the number of books on history in the library?
    A 110 Correct Answer Incorrect Answer
    B 128 Correct Answer Incorrect Answer
    C 108 Correct Answer Incorrect Answer
    D 98 Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Let the number of books on History, Polity and Geography be x, y and z respectively. Given, (x + y + z)/3 = 180 Or, x + y + z = 540 ….. (1) And, (x + z)/2 = 150 Or, x + z = 300 ….. (2) From equation 1 and 2 Number of books on polity = y = 240 Number of books on geography = 0.8 × 240 = 192 Therefore, number of books on history = 540 – (240 + 192) = 108

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