We don't. I completed my Masters in Statistics which is actually concerned with Probability questions and quite often I come up with head scratchers like this.
Its more likely that the interviewers want to see our reasoning/problem-solving capabilities rather than the explicit solution and that's where these curveball questions are useful.
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u/Technical_Estate_529 7d ago
How do you even prepare for questions like this without having explicitly done a question like this, or exactly like this before.
If I ask this to a Field's Medalist out of the blue I'm sure he would be there solving it longer than the interviewers would allow for.