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Jin314159

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Jin314159

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I would think 1/9 but i dont actually remember how to do probablility.

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Claude Bile

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1st person truthful, 2nd person truthful

1st person truthful, 2nd person lying

1st person lying, second person truthful

1st person lying, second person lying.

Work out how probable each possibility is, then work out which possibilities actually result in the second person claiming the first person is truthful. The rest should be easy.

Claude.

- #4

Jin314159

Here's what I did:

P (1st telling truth | second says first is telling truth) =

P (1st telling truth and second says first is telling truth) / [P(1st is telling truth and second says first is telling truth) + P(1st is lying and second says first is selling truth)]

= 1/3

P (1st telling truth | second says first is telling truth) =

P (1st telling truth and second says first is telling truth) / [P(1st is telling truth and second says first is telling truth) + P(1st is lying and second says first is selling truth)]

= 1/3

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Or they are telling true , or they are lying .Jin314159 said:

The answer is ( 1/3 * 1/3 ) / ( 1/3 * 1/3 + 2/3 * 2/3 ) = 1/5 .

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Fredrik

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Fredrik

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Suppose that you say "it's raining", and your neighbor says "what musky ox is saying is true".

Which ones of the following alternatives are*possible*, and which ones aren't?

1. You're telling the truth and he's telling the truth.

2. You're telling the truth and he's lying.

3. You're lying and he's telling the truth.

4. You're lying and he's lying.

Once you have figured that out, the rest will be very easy.

Which ones of the following alternatives are

1. You're telling the truth and he's telling the truth.

2. You're telling the truth and he's lying.

3. You're lying and he's telling the truth.

4. You're lying and he's lying.

Once you have figured that out, the rest will be very easy.

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The answer is not 1/3 . Let's follow the Claude's suggestion:Fredrik said:

Claude Bile said:Consider the 4 possibilities -

1st person truthful, 2nd person truthful

1st person truthful, 2nd person lying

1st person lying, second person truthful

1st person lying, second person lying.

The first pair of persons ocurrs with probability: 1/3 * 1/3 = 1/9

The second pair ocurrs with probability: 1/3 * 2/3 = 2/9

The third pair ocurrs with probability: 2/3 * 1/3 = 2/9

The fourth pair ocurrs with probability: 2/3 * 2/3 = 4/9

But, since the 2 persons agreed, we know only the first or the last case can be possible.

So, the probability we have a truthful pair of persons is (1/9) / (1/9 + 4/9) = 1/5.

This is classical Bayes.

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Fredrik

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Yes, you're right. I assigned probability 0 to the impossible "possibilities", but that's not the right way to do this.Rogerio said:The answer is not 1/3 .

...

But, since the 2 persons agreed, we know only the first or the last case can be possible.

So, the probability we have a truthful pair of persons is (1/9) / (1/9 + 4/9) = 1/5.

This is classical Bayes.

- #11

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Well, in fact, **you and Claude Bile** did the job !

I just followed yours suggestion...

I just followed yours suggestion...

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