Answers

2015-09-12T19:16:45+05:30
21 boys. 3:7 = 6:14 = 9:21; 9+21 =30. Therefore no.of boys = 21.
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2015-09-12T19:53:37+05:30
Let ratio constant be x  --
ratio of girls =3x
ratio of boys =7x
 
number of girls =30
number of boy = ?

ratio of girls + ratio of boys = number of girls 
            3x  +  7x               =  30
                                  10x = 30
                                      x =30/10
                                      x =3
Now, put the value of x ---
ratio of girls  =3x
                  = 3*3
ratio of girls =9
 
ratio of boys = 7x 
                  = 7*3
ratio of boys = 21     
                 
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30 is the total no of girls and boys together. so the ratio of girls + ratio of boys = total no. of students