We have a scale that relates the measures in a model with the measure in the real world.
The scale is 1 cm = 0.5 m, meaning that 1 cm in the model represents 0.5 m in the real world.
If we want to know what real world measure is representd by 8 cm in the model, we can use the Rule of three as:

We can see that 8 cm in the model represents 4 meters in the real world.
Answer: 4 meters.