The scale on a map is 1:25000. The lenght of a wall on the map is 3.2 mm. Find the actual length in metres.
Each 1 mm distance on the map represents 25,000 mm (= 2.5 meters) on the ground. If the length of some feature on the map happened to be 10 mm, you would set up a proportion like this, where x represents the actual length of the feature, in mm.
After solving for x in this equation, convert to meters.