【Question 10】
There is a "buy five get one free" soda, where every five empty bottles can be exchanged for a new bottle of soda. During a class gathering, a class drank 109 bottles of soda, some of which were obtained by exchanging empty bottles. How many bottles of soda did they initially buy at least?
【Analysis and Solution】
Equivalent substitution.
Since 5 empty bottles $=1$ bottle of water + 1 empty bottle; therefore, 4 empty bottles $=1$ bottle of water; so for every 4 bottles bought, one can drink 5 bottles of water;
$$
109 \div 5=21 \cdots \cdots 4 \text {; }
$$

The class leader just needs to buy $21 \times 4+4=88$ bottles of mineral water to ensure that everyone gets one bottle.