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Infosys Written Aptitude Interview Question

Infosys Written Aptitude Interview Question - 15 May

Alex speaks truth only once a day in a week. Below are a few hints for you:

First: Days are Sunday, Monday and so on.
Second: One day he says, 'I lie on Monday and Tuesday'.
Third: On the next day, he says, 'Today is either Thursday, Saturday or Sunday'.
Fourth: On the next day, he says, 'I lie on Wednesday and Friday'.

Can you identify the day on which he speaks truth?



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7 comments:

  1. He speaks truth every day... . Cozz its once a day

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  2. I don't think with the premise given, this situation is possible

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  3. The first day is Sunday, and he speaks the truth on Tuesday.

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  4. He starts on Sunday, and tells the truth on Tuesday.

    Here is the logic:

    If he is lying on the 1st day, he tells the truth on either Monday or Tuesday. If he is lying on the 3rd day, he tells the truth on either Wednesday and Friday. Both cannot be possible, therefore, he must be telling the truth on either the 1st or the 3rd day. This means that he is lying for sure on the 2nd day. It also means that he must be telling the truth on either Mon, Tue, Wed, or Fri, one of which must be either the 1st or 3rd day.

    If he is lying on the 2nd day, then that day must be either Mon, Tue, Wed, or Fri. This means the 1st day is either Sun, Mon, Tue, or Thu.

    If Sun, Mon, Tue or Thur must be the 1st day, we can prove that it cannot be Monday or Tuesday. He cannot tell the truth on either of those days because his first statement says that he lies on those two days. We cannot have them be a truth day and he also says he lies those days. Therefore, he can only start on either Sunday or Thursday.

    Thursday cannot be the first day, because we already established that Mon, Tue, Wed, or Fri must be either the 1st or 3rd day. Starting on Thursday makes that impossible.

    Therefore, we can only start on Sunday. We know he must tell the truth on either Mon, Tue, Wed, or Thurs, so he must start Sunday with a lie.

    (1st day) Sunday he lies and says he tells a lie on Mon and Tues. This means that Monday or Tuesday must be the truth day. The 2nd day must be a lying day, therefore, Tuesday is the truth day.

    (2nd day) Monday he lies and says the day is Thurs, Sat, or Sunday. That works out.

    (3rd day) Tuesday he tells the truth and says he lies on Wed and Friday. That works out.

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  5. I think he speaks truth everyday "once"

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