[lgarron]

Why wouldn't you be able to get the average? Return page count + total pages for each shelf and then take the ratio of "reduced" sums.

[michael_dorfman]

Re-read the comment. It discusses exactly what you are proposing.

[lgarron]

Oh, I see. I was struck by the direct phrasing of "You couldn't take the average number of pages", which made me parse the rest of the comment incorrectly.