Shannon made a scale drawing of a farm. The scale she used was 1 centimeter : 9 meters. The actual length of the goat pen is 18 meters. How long is the pen in the drawing?
To find out the length of the pen in the drawing, we need to use the given scale of 1 centimeter: 9 meters.
Since the actual length of the goat pen is 18 meters, we divide it by the scale factor of 9 meters: 18 meters / 9 meters = <<18/9=2>>2 centimeters.
Therefore, the length of the pen in the drawing is 2 centimeters. Answer: \boxed{2}.