The distance on the ground of 20 meters is shown on the plan with a 1 cm line, determine the scale of the plan.

To calculate the scale, you need to know the distance that is marked on the ground and know its analogue on the plan, the same distance if transferred to the terrain. In this problem, 20 meters on the ground correspond to 1 centimeter on the plan, respectively, this is the scale of the image, you only need to translate it into one common unit of measurement (centimeters).
1 meter = 100 centimeters.
20 meters = 2000 centimeters.
Accordingly, the drawing on the plan is made in a size ratio of 1: 2000.