Question 414633
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Hi
If the ratio of boys to girls in a class is 3:5
 3/5 = b/g = b/(C-b)
 3(C-b) = 5b
  3C = b(5+3)
  3/8C = b
fraction of the class who are boys is  3/(3+5) = 3/8

For example:  40 in a class:  3/8*40 = 15 boys (40-15 = 25 girls) 
  wihich gives the proportion: 3/5 = 15/25