An interview question

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Hi, Everybody,
Since I am having quite a few interviews recently.  I like to share with you the toughest question, just to prepare yourself for future interviews, or as a brain teaser.  You may contribute to my questions list by sending email to me: Matt.Damon5513@msn.com
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Question:
General Manager of a good company is trying to hire one executive assistant.  among the 100 applicants, they picked the smartest 3 through the first round of screening.
To determine which one will get the job, General manager invited 3 ladies to his office.  He showed 5 cards to them.  3 cards are blue, and 2 are white.
He said: "Now I am going to pin one card on the back of each of you."  And he pinned one blue card to each lady's back, then he hided the 2 white cards in his drawer.  "Now, all of you can see the color of the cards on others' back.  But I want you to tell me what color is the card on your own back."
All 3 ladies suspect that they got a blue card on the back.  But they just cannot give a valid reasoning...  30 minutes passed.  Finally one lady stood up, and said -- "I have a blue card on my back!"  Then she gave a very valid reasoning.
Now can you tell me what's the valid reasoning to reach here conclusion?