I want to point out the insanity of filtering on an interview question that has a low success rate. Presumably your candidate has a bachelor's degree. By this

I want to point out the insanity of filtering on an interview question that has a low success rate. Presumably your candidate has a bachelor's degree. By this point in their life they've answered thousands and thousands of questions. They taken countless exams, and there is solid aggregate data which summarizes their performance (GPA, standardized tests, certificates, etc). By filtering on a few challenging questions, the interviewer is throwing these statistics in the trash and rolling the dice to disambiguate the best and worst candidates. What do you think the variance is on a five question exam? Not great. Rather than try to assess decades of expertise with five questions, I think you are better served by asking questions which pull in orthogonal information. Do the candidates career goals align with the companies? Do they seem pleasant to be around (also high variance, but orthogonal). Are they enthusiastic about the position?