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Math Help - Logic riddle

  1. #1
    Member Rimas's Avatar
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    Logic riddle

    kerry and Kelly are twins.One of them lies on mondays, Tuesdays and Wednesdays and tells the truth on the other days of the week> The other lies on Thursdays,Fridaysa and Saturday and tells the truth on the other days of the week>oon which day of the week did they have this conversation?


    Kerry: I lie on Saturdays
    Kelly: I will lie tomorrow
    Kerry: I Lie on Sundays
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Rimas View Post
    kerry and Kelly are twins.One of them lies on mondays, Tuesdays and Wednesdays and tells the truth on the other days of the week> The other lies on Thursdays,Fridaysa and Saturday and tells the truth on the other days of the week>oon which day of the week did they have this conversation?


    Kerry: I lie on Saturdays
    Kelly: I will lie tomorrow
    Kerry: I Lie on Sundays
    Since neither lie on Sundays, Kerry is lying today, and so must tell the
    truth on Saturdays, which means Kerry lies on Mondays, Tuesdays and
    Wednesdays, and today is one of these days.

    So when Kelly says I will lie tomorrow she is telling the truth as the
    twins never lie on the same day.

    So the day you seek is a day on which Kelly tells the truth (one of
    Sunday, Monday, Tuesday, Wednesday), but will lie on the next day
    (one of Thursday, Friday, Saturday).

    RonL
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  3. #3
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    Quote Originally Posted by Rimas View Post
    kerry and Kelly are twins.One of them lies on mondays, Tuesdays and Wednesdays and tells the truth on the other days of the week> The other lies on Thursdays,Fridaysa and Saturday and tells the truth on the other days of the week>oon which day of the week did they have this conversation?

    Kerry: I lie on Saturdays
    Kelly: I will lie tomorrow
    Kerry: I Lie on Sundays
    Hi,

    both twins are telleing the truth on Sundays. So the last sentence is a lie. Thus the first sentence must be a lie too.
    Therefore the second sentence must be true.

    Now look who is telling the truth at Saturdays (remember: The first sentence was a lie!)

    This conversation took place at a Wednesday, when one is telling the truth that she'll lie tomorrow and the other tells a lie (she must be in her lying mood).

    EB
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