Question

the ratio of boys to girls in Ms.Smith`s class is 3 to 4.If there are 12 boys,how many girls are there?

Answer 1

The question says boys/girls = 3/4 so if there are 12 boys in the class you can say:
3/4 = 12/? 
and ? is the number of girls which is equal to:
? = 12*3/4 = 9 
so if there are 12 boys in the class we will have 9 girls in the class :)))
i hope this is helpful 
have a nice day