6 feet
20/15= 1.3 8/1.3=6.1 round to the nearest tenth and BOOM 6 feet

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 

The way to solve this is to do 5 1/5 divide by 2.5.
2.08 minutes