In a seventh grade class, 14 out of 30 students are boys. In simplest terms, what is the ratio of the number of boys to the number of girls in this grade? A. 7:15
B. 15:7
C. 7:8
D. 8:15

They already give you the number of boys {14}, so all you need to do is subtract 30 - 14 which will give you the ratio of girls.

It does this because 14 takes up of the seventh grade class.

  So your answer is going to be 14:16 un-simplified.

  However, since they ask you to put it in the simplest terms, we are going to simplify

  You can do this by dividing both numbers by 2.

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or