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　　　The story of the ice-cream sellers

    Have you ever noticed that you often find the same kinds of stores together on the same street?  Why are they together? Economics gives us one explanation…

　Imagine a beach a half mile long, full of sunbathers.  The sun is shining, and the water is warm.  An ice-cream seller called George arrives.  Where does he put his ice-cream cart?  Obviously, he puts it in the middle of the beach, where the sunbathers can easily walk to him and buy some ice-cream.  

     Later that day, a second ice-cream seller, Georgina, arrives.  George and Georgina talk for a while and decide that the best solution is this: (            ), and they each put their ice-cream cart in the middle of their half.