Thanks to Eric for point out that the two marbles solution I presented previously isn’t optimal. I’m pretty confident that allowing the first pass windows size to vary will fix it up, but haven’t found the time to site down and work it through.

On the other hand, it crossed my mind this morning that the question didn’t state what we should optimise for. I imagine the number of marble drops is what’s expected, but if you took into account the running up and down stairs involved that may well affect the solution…

Hmmmm…