Question
The average number of books on history, polity and geography in a library is 190, and the average number of books on history and geography is 160. 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 = 190 Or, x + y + z = 570 ….. (1) And, (x + z)/2 = 160 Or, x + z = 320 ….. (2) From equation 1 and 2 Number of books on polity = y = 250 Number of books on geography = 0.8 × 250 = 200 Therefore, number of books on history = 570 – (250 + 200) = 120
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