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Discrete Mathematics

GATE CSE 2015 Set 3 · Q24

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In a room there are only two types of people, namely Type 1 and Type 2. Type 1 people always tell the truth and Type 2 people always lie. You give a fair coin to a person in that room, without knowing which type he is from and tell him to toss it and hide the result from you till you ask for it. Upon asking, the person replies the following “The result of the toss is head if and only if I am telling the truth.”

Which of the following options is correct?

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Solution

Step-by-step verified solution

Official answer: A) The result is head

Discrete Mathematics

Let HH denote the proposition "The result of the toss is head" and TT denote the proposition "The person is telling the truth".

The person makes the statement:

S≡(H  ⟺  T)S \equiv (H \iff T)

We analyze both cases:

  1. If the person is Type 1 (truth-teller): Here, TT is True, so the statement SS must be True.

    S=(H  ⟺  True)≡H=TrueS = (H \iff \text{True}) \equiv H = \text{True}

    Thus, the result is Head.

  2. If the person is Type 2 (liar): Here, TT is False, so the statement SS must be False. $$S = (H \iff False\text{False}) \equiv

eg H$Since Since SisFalse, is False, eg HisFalse is False \implies H$ is True. Thus, the result is Head.

In either case, the result of the coin toss is definitely head. Therefore, (A) is correct.

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This Discrete Mathematics previous year question is Q24 of the GATE CSE 2015 Set 3 paper. Find more GATE CSE PYQs on Discrete Mathematics in Engineering Mathematics, or browse all GATE CSE PYQs.

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Discrete Mathematics in GATE CSE

Discrete Mathematics has 50 GATE CSE PYQs across 23 papers from 2011 to 2026. 18 of them are 1-mark MCQs like this one, and GATE CSE 2015 Set 3 has 2.

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GateAI solution cardGATE CSE 2015 Set 3 · Q24 · MCQ

In a room there are only two types of people, namely Type 1 and Type 2. Type 1 people always tell the truth and Type 2 people always lie. You give a fair coin to a person in that room, without knowing which type he is from and tell him to toss it and hide the result from you till you ask for it. Upon asking, the person replies the following “The result of the toss is head if and only if I am telling the truth.” Which of the following options is correct?

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.