Find Which of Three People Is Telling the Truth
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Key Takeaway
Yesterday's Quiz 4 answer is 33. Today's Quiz 5 is a logic puzzle: three people each always tell the truth or always lie, and you must work out who is telling the truth. Today's answer goes up tomorrow in Quiz 6.
What was yesterday's answer?
Quiz 4 asked for the number after 2, 3, 5, 9, 17. The answer is 33.
| Previous number | 2 | 3 | 5 | 9 | 17 |
|---|---|---|---|---|---|
| ×2−1 | 3 | 5 | 9 | 17 | 33 |
The differences also work: 1, 2, 4, 8 double each time, so the next difference is 16, and 17+16=33. Both methods point to the same answer.
What is today's puzzle?
A, B, and C are each either someone who always tells the truth or someone who always lies. The three said the following.
| Person | What they said |
|---|---|
| A | "B is a liar." |
| B | "C is a liar." |
| C | "A and B are both liars." |
- Question: Who is telling the truth? (It could be one person or several.)
- Difficulty: ★★★☆☆
- Suggested time: 5 minutes
Where should you start?
Start by assuming things about C, whose statement mentions the most people. Suppose "C is telling the truth," check whether it clashes with what the other two said, and if it does, move on to the opposite case. Writing each person's truth or lie in a table for each case cuts down on mistakes.
When can you check the answer?
The answer and explanation go up tomorrow (October 18) morning, at the very top of the Quiz 6 post. The case-by-case reasoning will be laid out in a table too.
What practice do truth-teller and liar puzzles give you?
Making an assumption and discarding it when it leads to a contradiction is the basis of reasoning from conditions. Mensa Korea describes its online brain test as looking at cognitive abilities including logical thinking (checked 2026-10-13). Mensa International's standard for joining is a top 2% score on an approved intelligence test, and this puzzle is practice of the same type.
Frequently Asked Questions
What is the answer to Quiz 4?
33. Each number is the one before it times 2, minus 1, so 17×2−1=33.
When does the Quiz 5 answer come out?
It goes up with the explanation at the very top of the Quiz 6 post, published the next day.
Could all three people be liars?
The conditions leave that possibility open. Work it through yourself and the answer comes out as exactly one.
Sources & Verification Date
- Mensa International membership requirements guide · Verified
- Mensa Korea online brain test introduction · Verified
All materials have been verified from official announcements and may have changed since verification.