$5 、$ a bottle of cola costs 2.5 yuan, 3 empty bottles can be exchanged for one more bottle of cola. With 30 yuan, the maximum number of bottles of cola you can drink is $\qquad$ bottles.
【Note】This question is ambiguous. If you can borrow a bottle from someone else, the answer should be 18; if not, the answer should be 17.

【Analysis】One, you can borrow bottles from others
According to the problem, 3 empty bottles $=1$ bottle of cola + 1 empty bottle
So, in fact, 2 empty bottles $=1$ bottle of cola
This means, spending 5 yuan, you can buy 2 bottles of cola, and actually drink 3 bottles of cola (after drinking 2 bottles, you have 2 empty bottles, borrow 1 empty bottle, exchange for 1 bottle of cola, drink it, and return the empty bottle)
Therefore, with 30 yuan, you can drink up to $30 \div 5 \times 3=18$ bottles of cola
Two, you cannot borrow bottles from others
30 yuan can buy $30 \div 2.5=12$ bottles of cola
12 empty bottles can be exchanged for $12 \div 3=4$ bottles of cola
4 empty bottles can be exchanged for 1 bottle of cola
In the end, you drink $12+4+1=17$ bottles of cola, and have 2 empty bottles left.