I've also been really confused by this vignette...
Originally I thought the west 2nd floor part of the section should allocate for the 30" joist and the 24" deep duct in the corridor, despite the fact that the section cut is through the 16" deep duct (looking for the biggest duct/joist combination).
After reading this thread and taking another look at the vignette, it looks like the corridor joist is actually 24" as it originates at the North bearing wall (where all other joists are 24")?
However, even if this is the case, the space needed for the joist and duct would still be 24"+24" = 48" which is even larger than the 30+16 combo...
I test next week and this has really confused me
Should the plenum for the 2nd floor be 24"+24"+8"? or 30"+24"+8"? or as Dorf shows 30"+16"+9"?
I have been looking at it so long and am not sure what I am missing! Any ideas?