Payment for Class B purchase of mineral water: 35*2*0.8=56.

Payment for purchasing Class A mineral water: 60.8

So pay less: 60.8-56=4.8 (yuan)

2. If X bottles are purchased on the first day and (35-x) bottles are purchased on the second day, then:

Because Class A exceeds 15 bottles twice, then: 2*0.8*35=56 yuan.

The first time I bought more than the second time, so I bought more than 15 bottles for the first time and less than 15 bottles for the second time.

So what? 2*0.8*x+2*(35-x)=60.8

Solution: x=24

So I bought 24 bottles on the first day and 1 1 bottle the next day.