Buy five bottles first. Finished, five empty bottles and five lids. Then change one bottle to four lids. There will be six empty bottles and a lid after drinking. There will be three empty bottles after drinking. There will be four lids before adding one lid. There will be four empty bottles and a lid after drinking. Then, four empty bottles will be replaced by two bottles of soda. After drinking, there are four lids and an empty bottle, so you can exchange four lids for a bottle of soda, and after drinking two empty bottles, you can exchange one bottle of soda, so there are only two lids left and you can't change them. According to the calculation, you can drink 15 bottles of soda.