Wow, in case anyone's interested in an alternate solution, here's how I did level 27:
-bought disco immediately and advertised
-sat 4 people, upgraded disco
-earned money to buy drink stand
-upgraded disco fully, advertising as soon as last advertisement was gone
-upgraded drinks fully (remembering to keep drinks in stock if people want more, but only as I was already serving last possible drink from drink stand)
-stopped advertising disco when I got to 15-18 people or so (should have 1 or 2 ads left)
-bought meat stand, upgraded fully, advertised
-advertised drinks if I had 2 ads left instead of 1 when I had 15-18 disco patrons
-bought souvenir stand, upgraded fully (note that this was the last thing I did, and no souvenirs were sold)
this is all dependent on having bought all previous upgrades. did not buy server or maintenance, did not upgrade storage or water or delivery
So this took me like a month to figure out, and I modified my style from libby's incredibly helpful post. My time was 5:25, so it's crunchy, dependent on luck, and probably not achievable unless you can chain like crazy
one little hint I picked up: if you see that someone at the disco is running out of patience but you see that you're going to be able to pick up the "magic 3 chain" if you just wait a second for the customers to get over there, start to go to the disco by clicking on the impatient person but at the last second click on her again so that you're really close, but haven't reached her yet. If you time it just right, you'll be able to wait until the other 1 or 2 customers get there to chain 3 and you won't lose the impatient customer. Obviously this means that you can't have another task waiting so that you drift off to do that before you can get to the disco customers. Sometimes I had to go back and unclick waiting tasks to do this successfully.
Using my scenario I have to ignore people waiting to be seated at times, and had to clean up in chains like crazy, but it helps to remember people waiting in line will wait forever because their patience meter hasn't started yet.
Still haven't got expert on levels 38 and 40; read posts and tried them, was unsuccessful. I'll keep trying. Good luck to all, and special thanks to libby - I wouldn't be posting here if not for her.