"First, you have 35 total pens in the box. 15 are green. to find how many red pens you need to find the difference between 35 and 15 to get how many red pens there are. (35-15= 20) Therefore, there are 20 red pens. The ratio needs to be green pens to red pens. So, there is 15 green and 20 red ( 15/20 or 15:20) Simplify that by 5 and you get 3/4 or 3:4. "