理性投资

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Originally uploaded by Aimee & Aidan

米米和弟弟一起去植物园看火车，拿了一块钱要自己买甜点。来了个叔叔，拿着两个两毛五的硬币说，我们换一下吧，你看，两个钱换一个钱。米米想了半天，摇摇头。

回头妈妈问米米，怎么没换呀。米米说，"Because I don't know the number, I don't know if it is a good deal."