I used in my family to play with my own kid (who is 6) asking him similar questions. His brain looks like is being shaped in a right direction, even though he doesn't know precise answers, it's all about the approach.
This case or similar to this one is about how you find the route causes or the main parts, which impact the result. What is important when answering this question is how you think. Even if you have no clue on roads density.
I give you an analogy. How many bulbs one may put on a Christmas tree at max? For answering this question you need to understand:
* Number of tree branches
* Average lenght per branch
* Average size or diameter of the bulb
* And then - use your judgement to understand if the tree will be staying still
In answering your questions, the approach may be similar, although there is more than 1 approach:
1. Top down:
* Size of Spain (in km)
* Judgemental part of country covered by roads (in cities and outside)
* Multiple one by another
2. Bottom up:
* Number of residents & guests in the country (population)
* Part lives in cities and outside
* Time spent per 1 person in the roads
* Average speed of trip
3. Construction based
* Norm of construction of roads per year since 1900
* Annual growth (linked with: a) car growth, b) infrastructure spending growth or c) productivity growth
* Add some random roads among main hubs and junctions
* Imagine that the entire territory of the country consists of roads, then - start taking off the number areas, which belong to cities, historical places, parks, mountains, etc. Because in fact - one can try to go through absolute variety of roads (meaning that if there is no road - it doesn't mean one can't go there).
5. Car based
* How many cars are in Spain
* How many KM are driven by car
* How often cars go the same/different direction
* What is the density on the road
* How large part of road had never being driven on (or let's say for the past x years)
PS. you may find some other x alternatives of answering this question. Pls don't be always simple and straight, show some of the imaginations. Esp in infrastructure projects in Spain!