The scale is the relation between the distance on a map and the distance on the field :

For example a scale 1:25,000 represents :

That means 1mm on the map is worth 25,000mm in real life (25m)

It also means 1 cm on the map is worth 25,000cm in real life (250m) So 4 cm make 1km for real.
Example 1 : We have measured a distance of 5.5cm on a map scale 1:50,000. What is the real distance ?

Solution 1 : You have to use a cross multiplication
5.5cm correspond to X m

1cm is 50,000cm = 500m

Solution 2 : 1:50,000 means that 1mm corresponds to 50m on the field, so :

55mm x 50m = 2 750m

Exemple 2 : How many cm do you need to represent 18km on a 1:250,000 scaled map ?

X cm are 18km

1cm is 250,000cm = 2.5km