Explanation
The length of the back patio in real life is 27 meters long.
The length of the drawing is 3 millimeters.
The required scale is 3 millimeters : 27 meters.
To lowest ratio, divide through by 3
3/3 : 27/3 = 1 millimeter : 9 meters
Therefore, the scale Camilla used was 1 millimeter : 9 meteres