Assumptions need to be stated or you're solving only a discreet part of the problem! Try this, see if you get another deadpan assumption.
A solar system has 3 planets in concentric orbit. PlanetZ is the farthest with an orbit diameter of 9. PlanetY has an obit diameter one greater than PlanetX. What is the orbit diameter of PlanetX?
I mean, the model is intended to help the user, not fight against the user trying to break it. IMO, it is reasonable for such model to default on making assumptions and going forward as long as the assumptions are clearly stated.