5/(5/3)=3 yuan A used 9 yuan to buy coke, and B used 6 yuan.

So C should give A 9-5=4 yuan.

2. Changfa Supermarket: 230*0.8= 184 yuan.

Dongfeng Supermarket: 230-50= 180 yuan

Shopping Center: 230*0.9-20= 187 yuan

Therefore, it is most suitable to buy in Dongfeng supermarket.

3. First find the side length of the original square = 15/3=5.

So what is left is a rectangle with a length of 5 decimeters and a width of less than 2 decimeters.

So you can cut a circle with a maximum diameter of 2 decimeters, or you can cut two circles.