Final answer:
The length of the model is 3 km.
Step-by-step explanation:
To find the length of the model, we need to use the scale of 1:5. This means that each unit on the model represents 5 units in the actual bridge. Since the actual bridge is 15 km long, we can find the length of the model by dividing 15 km by 5. This means every kilometer on the model corresponds to five kilometers in reality. Applying the same scale factor to the width, the model's width would be 12 km / 5 = 2.4 km. Thus, the scaled-down model accurately represents the proportions of the actual bridge, maintaining a 1:5 ratio. Therefore, the length of the model is 3 km.