I've been working on this for hours, and I desperately need help.

I'm simplifying the spreadsheet immensely, but the heart of my problem is
this: In my spreadsheet of apartment buildings, the # of apartments in a
building is in column AD, and rent per apt. unit is in column BB:

AD BB
228 800.00
450 880.00
964 870.00
290 760.00

I've been asked to create a summary table which calculates the average rent
per unit for various ranges -- for instance, the average rent per unit for
buildings with 100 to 299 apartments, 300 to 499 apartments, etc.

So I need to filter rows in column AD to show just the buildings within a
range (100-299 apartments, for example), then sum column BB for only those
AD-filtered rows, then average column BB for only the AD-filtered rows. I've
read hundreds of posts and tried dozens of combinations of SUMIF, COUNTIF,
and SUMPRODUCT, and I'm just not getting there.

Please save me!!!

A million thanks.