Arcadis 25 km long on map it is 5cm long. Calculate the scale used?

We know that $1 \text{ km} = 1000 \text{ m}$ and $1 \text{ m} = 100 \text{ cm}$.
Therefore, $1 \text{ km} = 1000 \times 100 \text{ cm} = 100,000 \text{ cm}$.

The real distance is $25 \text{ km}$.

$$25 \text{ km} = 25 \times 100,000 \text{ cm} = 2,500,000 \text{ cm}$$

The map distance is $5 \text{ cm}$.
The real distance is $2,500,000 \text{ cm}$.
The scale is given by:

$$\text{Scale} = \text{Map distance} : \text{Real distance}$$ $$\text{Scale} = 5 \text{ cm} : 2,500,000 \text{ cm}$$

Divide both sides of the ratio by 5:

$$\frac{5}{5} : \frac{2,500,000}{5}$$ $$1 : 500,000$$