Here's a different test that would also work, if I'd previously memorized the answer: "5 decimal digits of pi starting at the 243rd digit!" Although it might be too obvious, and now that I've posted it here, it wouldn't work in any case.

Yes, that would be too obvious. And no, I'll never get those hours of my life back.

The previous memorization isn't too important. You need him to be fast. You can then put him on hold while you googled.