Question
The average number of books on history, polity and
geography in a library is 150, and the average number of books on history and geography is 140. 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 = 150 Or, x + y + z = 300 ….. (1) And, (x + z)/2 = 140 Or, x + z = 280 ….. (2) From equation 1 and 2 Number of books on polity = y = 170 Number of books on geography = 0.8 × 170 = 136 Therefore, number of books on history = 450 – (170 + 136) = 144
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