What is the probability of chancing on...

That might work in theory, but rarely in reality.

After all, what is the probability of, in taking a job interview written exercise, chancing on a topic/scenario one happens to have committed a pre-written article to memory on?

Keep in mind that the setup for a written exercise is often very specific, not infrequently with unusual, if not unique, circumstances, where the candidate is asked to analyze the parameters before committing to a certain course of action with proper/adequate justification.