PROBLEM 263

My wife and I were recently invited to a dinner to be held on 4-5-2007.  In Europe, where they write the day first, followed by the month, this would refer to 4 May 2007; in the USA, where the month is written first, this would refer to 5 April 2007.  Since our hosts are recent arrivals from Europe, we weren't sure when the dinner would take place.  A quick phone call cleared up the ambiguity.  

What is the probability that a randomly chosen date is ambiguous?

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ã2007 Alberto L. Delgado