First, convert the 90 feet of the real plane to inches so that you can find the scale: 90 x 12 = 1080 Now, divide the inches of the real wing span by the inches of the model wing span: 1080/18 Now you want to simplify. The easiest way to start is dividing the top and bottom by 2: 540/9 Now try dividing the top and bottom by 9: 60/1 So your scale is 1 inch of model airplane to 60 inches of actual airplane.