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Hi,
a science class has a ratio of 3 boys to 2 girls.
 B/G = 3/2   | Cross Multiplying 
  3G = 2B
   G = (2/3)B
Question states***
what is the largest class that could be formed with less than 40 students?
 B + (2/3)B = 40   for ex    
          B = (3/5)*40   18 and 12   
          B = (3/5)35    21 and 14   35 the largest class to maintain the ratio