So there is 18 pens right.
12 of them are blue
The rest are red
So thats 12:6
If you want red to be one divide by 6
What you do to the top you do to the bottom
12/6= 2
So for every 2 red you have 1 blue
2:1
I have straight A's in class i'm kinda like a math nurd...
Hope this helps ツ