10 yuan: buy 5 bottles.

5 bottle caps, 5 empty bottles: 3 bottles.

After drinking, it's left.

4 bottle caps, 4 empty bottles: 3 bottles.

After drinking, it's left.

3 bottle caps and 3 empty bottles: 1 bottle,

There are 4 bottle caps and 2 empty bottles left: change 2 bottles.

After drinking, there are two bottle caps and two empty bottles left. Change: 1 bottle.

*** 15 bottles.