Question 56

Logical Reasoning Puzzles Medium

A remote island has a unique social structure. Individuals are either 'Truth-tellers' (who always speak the truth) or 'Tricksters' (who always lie). You encounter three inhabitants: X, Y, and Z. X says: 'Y is a Trickster.' Y says: 'Exactly one of us is a Truth-teller.' What can you definitively conclude about Z?

(A) Z is a Truth-teller
(B) The information is insufficient to determine Z's identity
(C) Z could be either a Truth-teller or a Trickster
(D) Z is a Trickster
View Dynamic Solution & Explanation
Correct Solution: Option C

Step-by-step Solution:

\[ \text{Let }T=\text{Truth-teller},\quad L=\text{Trickster (liar).} \] \[ \text{Statements:}\quad X:\ "Y\ \text{is a Trickster}"\qquad Y:\ " \text{Exactly one of }(X,Y,Z)\text{ is a Truth-teller.}" \] \textbf{Case 1: } Assume \(X\) is \(T\).\\ Then \(X\)'s statement is true, so \(Y\) is \(L\). Since \(Y\) is lying, the statement “exactly one of \((X,Y,Z)\) is \(T\)” is false. But \(X\) is \(T\), so to make “exactly one” false, \(Z\) must also be \(T\) (giving 2 truth-tellers). Hence in this case \(Z\) is \(T\), i.e. \(Z\) has the same type as \(X\). \medskip \textbf{Case 2: } Assume \(X\) is \(L\).\\ Then \(X\)'s statement is false, so \(Y\) is \(T\). Since \(Y\) is telling the truth, “exactly one of \((X,Y,Z)\) is \(T\)” is true. But \(Y\) is the single truth-teller, so \(X\) and \(Z\) must both be \(L\). Hence in this case \(Z\) is \(L\), i.e. again \(Z\) has the same type as \(X\). \bigskip \[ \boxed{\text{Therefore }Z\text{ is of the same type as }X\ (\text{both Truth-tellers or both Tricksters}).} \]