Question
The average number of books on history, polity and
geography in a library is 120, and the average number of books on history and geography is 130. 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?Solution
Let the number of books on History, Polity and Geography be x, y and z respectively. Given, (x + y + z)/3 = 120 Or, x + y + z = 360 ….. (1) And, (x + z)/2 = 130 Or, x + z = 260 ….. (2) From equation 1 and 2 Number of books on polity = y = 100 Number of books on geography = 0.8 × 100 = 80 Therefore, number of books on history = 360 – (100 + 80) = 180
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