The key idea of the truth teller identification in competitive exams is to test one's logical ability to answer questions and its usage is increasing day by day, It is the way to test your ability to identify the level truth or lie from many given inferences to draw some conclusion.From the given set of statements by few persons, has to draw a conclusion.
Truthteller, Lie Detector.
Types of people in Truth-teller and lie detector problems:
Liar: A person who always lie.
Truth Teller : A person who always says the truth.
Alternator: A person whose answers always alters in a step fashion(example: if he says 3 statements, his statements' truthiness can be either T, F, T or F, T, F)
One more point to Note:
"I'm a Liar":
It looks so simple but there is a lot in it, A liar cannot say this(because it is the truth) and a truth-teller cannot say this(As it is a lie), only an Alternator can say this statement.
When we get the statements from multiple persons, we may not easily distinguish them , the only way is to apply "Benefit of Doubt".
The benefit of Doubt: means assuming that the person is a truth teller and check other statements if no contradiction has emerged then the assumed assertion is true.
Amar visited a city and asked two persons, Rama and Krishna of whom one is a truth teller and other is a liar-in which direction Bombay was.
They replied as follows.
Krishna is a Liar.
Bombay is in the east.
I am not a liar.
Bombay is in the west.
First, give the Benefit of Doubt to any one of them,
I am giving Benefit of Doubt to Rama,=>Statements by Rama are true.
Krishna is a Liar and Bombay is in the west are true.
When we inference Krishna 's statement,
He says: I am not a liar. (Assuming this as False)
Bombay is in the west(This could also be false as he is a liar).
We can Drag the solution is "EAST"
Next give the Benefit of Doubt to Krishna,=>Statements by Krishna are true.
I am not a liar, Bombay is in the west are true.
when we inference Rama's statements,
He says: Krishna is a liar(Assuming this as False)
Bombay is in east(This could be false as he is a liar)
We can Drag the solution is "WEST".
Here we have two possible inferences which could be possible so we can derive solution "Bombay is either in East or in West.
Truthteller and lie detector
There are three creeds - truth tellers, liars, alternators.A and B belong to different creed among the three.Their replies for "Who is the Engineer?" are as follows:
I am not the Engineer.
I am a Liar.
I am not the Engineer.
A is not an alternator.
By observing the rules, we can easily say that A is an Alternator(as a Truth teller or a Liar can't use the statement "I'm a Liar")(False statement)
so 1st statement of A is a True statement(since alternator replies are complimentary each time)
I am not the Engineer (T)
I am a Liar(F)
A is an Alternator but B's second statement is "A is not an Alternator"
so B can be either Alternator or a liar.
As we can have only one from each creed, B cannot be an alternator.(With no contradiction)
I am not the Engineer.(F)
A is not an alternator.(F)
Conclusion: B is a Liar, A is an Alternator.
so B is an Engineer.
If you have any queries or doubts, feel free to comment ?