In an examination the average of all students in class A is 68.4 and that for students in class B is 71.2 . If the average of both the classes combined is 70, find the ratio of the no. of students in class A to the no. of students in classB . 
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Let the number of students in class A = n1Let the number of students in class B = n2We have,average marks of class A = 68.4sum of the marks of all the students in class Anumber of students in class A = 68.4sum of the marks of all the students in class An1 = 68.4sum of the marks of all the students in class A = 68.4 n1    .....1average marks of class B =71.2sum of the marks of all the students in class Bnumber of students in class B = 71.2sum of the marks of all the students in class Bnumber of students in class B = 71.2sum of the marks of all the students in class Bn2 = 71.2sum of the marks of all the students in class B = 71.2 n2   ........2Let average marks of both classes = 70sum of the marks of all the students in class A + sum of the marks of all the students in class Bn1 + n2 = 7068.4 n1 + 71.2 n2n1 + n2 = 7070n1 + 70n2 =68.4 n1 + 71.2 n21.6 n1 = 1.2 n2n1n2 = 1.21.6 = 34 

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sorry for that                                                    
  No.class A/No.class B=1.2/1.6=12/16=3/4
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