Mathematical principle: 4 empty bottles are known = 1 bottle of beer (no bottle)+1 empty bottle;

Push out: 3 empty bottles can drink 1 bottle of beer,

Then, if you buy 15 bottles of beer, you can drink 15+ 15÷3=20 bottles of beer.

Operation plan: ① buy 15 bottles and drink 15 bottles;

(2) borrow an empty bottle *** 16 for 4 bottles of beer and drink the remaining 4 bottles;

③ Switch to 1 bottle of beer and return the empty bottle to others after drinking.

* * * Drink 15+4+ 1=20 bottles of beer.