The Next Gatekeeper

Logic Level 1

Before you solve this question, solve The Two Gatekeepers

Before you, Merlin and your friend, Albert reach the real exit, now you see two doors guarded by one gatekeeper. You know that you can't fight him and win (he is HUGE). You know that one door leads to the exit, and the other leads to death (again?), but you don't know which is which. You don't know whether he lies or he tells the truth. You can only ask ONE question

Now Albert starts crying. "There was two. Now there is ONE only!" he said, and then continued crying.

You try to think of a question (while ignoring the cries). Does a question exist so that you can leave peacefully?

No Yes

Hua Zhi Vee by Hua Zhi Vee , 16, Malaysia

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2 solutions

Hua Zhi Vee
Nov 25, 2017

Point to a door and ask: "If I asked you 'Will that door lead to death?' will you answer yes?"

If he answers no, you go to that door. If he answers yes, you go to the other door.

Let us analyse the answers that he will give.

→→ If the door does lead to death,

Case 1 → He is truthful.

If you asked him 'Will that door lead to death?' he will answer yes. He is truthful so he will answer YES

Case 2 → He lies.

If you asked him 'Will that door lead to death?' he will answer no. He lies so he will answer YES

→→ Now, if the door doesn't lead to death,

Case 1 → He is truthful.

If you asked him 'Will that door lead to death?' he will answer no. He is truthful so he will answer NO

Case 2 → He lies.

If you asked him 'Will that door lead to death?' he will answer yes. He lies so he will answer NO

Hence, if he answers no, you go to that door. If he answers yes, you go to the other door.

Q . E . D . \huge\mathrm{Q.E.D.}

10 Helpful 4 Interesting 13 Brilliant 0 Confused

I have another possible answer, quite similar to yours but more similar to the first alternate riddle. I would ask "If your role was reverse(i.e truth-teller becomes a liar and vice versa), then which door would you point to if I asked you 'which door leads to the exit'?

The logic is even though there is only one guard, but you still do not know what position he holds.

Case 1 -> The guard is a truth-teller. This means he would assume the position of a liar and as the liar will point to the door of death when asked which door leads to the exit, the truth-teller will point towards the door of death.

Case 2 -> The guard is a liar. This means he would assume the position of a truth-teller and as the truth-teller will point to the door to the exit when asked which door leads to the exit, he points towards the door of death.

From analyzing the former and latter cases, we can come to a deduction that the door which leads to the exit will be the door which is not pointed at.

Sree Narayanan - 1 year, 8 months ago

Yes it is correct! Thanks for sharing your solution!

Hua Zhi Vee - 1 year, 7 months ago
Peixin Zhou
Jul 27, 2020

"If I ask you which door leads to the exit, what will you say?"

Go through the door that he shows you.

0 Helpful 0 Interesting 0 Brilliant 0 Confused

What if he lies?

Elijah C - 5 months ago

Yeah, What if he lies?

Ash Ketchup - 1 month, 4 weeks ago

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