Math problem

I am nowhere near as good at math as I would like to be. I looked at the following problem for a couple of minutes and I simply do not know either the correct answer or the methodology for how to derive the answer:

What number makes this grouping logical:

21, 8, 46, ___, 2, 92, 16, 42

Please let me know how to derive the solution.

I found this problem here.

Stop the presses! I just figured it out. Cute. It took me about ten minutes. A hard-core "math guy" probably could have done it in less than a minute if not almost instantly.

Now... here's the hard problem... design a computational algorithm that would have been able to discover the solution methodology.

-- Jack Krupansky