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.