Question
There are three classes P, Q and R. The average marks of
class P is 75. The average marks of class Q is 80. The average marks of class R is 85. The average marks of class P and Q is 77, the average marks of class Q and R is83. Find the approximate average marks of three classes.Solution
By allegation method: 
So the ratio of students in class P to Q comes out to 3 : 2 & the ratio of students in class Q to Ris 2 : 3. Ratio of students in P, Q and R = 3 :2 : 3 So, average of all the classes = `(75xx3 + 80 xx2 + 85xx3)/(3+2+3)` = `(225+160+255)/8` = 80
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