3:1 ratio works well for jets. Ex. If you need to lose 10,000 feet, you'd start down 30 miles from the fix. Vertical rate is GS dependent, if they give you GS, you can calculate time to fix, then divide altitde to lose by time to the fix.
If the interviewer is just checking basic mental math, just answer the question and don't try to second guess. It's rare to find a multi variable mental math problem in an interview so don't over think it. These questions are also a good segue into discussions about Econ decents vs. Crossing restrictions, etc.
Practice base 6 math, it comes in handy in an interview.
If they have a sense of humor, you might try - auto pilot off, FLCH, green arc on the fix.