Amodel airplane is built with a wing span of 18 inches the actual wing span of the airplane is 90 feet find the scale

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.

Equatorial triangle and obtuce angle