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.
=
or