The die.. problem

A person speaks truth 3 out of 4 times. A fair 6-sided die is thrown and the person reports that it is a 6. What is the probability that it was actually a 6?

1 5 \frac15 3 2 \frac32 1 2 \frac12 3 8 \frac38 3 4 \frac34

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2 solutions

Pop Wong
Mar 22, 2021

We just need to justify the person is telling truth or not, and P r ( Pr( this person tells truth ) = 3 4 ) = \boxed{\cfrac{3}{4}}

Some may use below table to find out the solution:

Pr Rolled 6 Rolled Not 6
Tell Truth 3 24 \color{#3D99F6}\cfrac{3}{24} 15 24 \cfrac{15}{24}
Tell Lies 1 24 \cfrac{1}{24} 5 24 \color{#3D99F6}\cfrac{5}{24}

They suggest the prob. = 3 24 3 24 + 5 24 = 3 8 = \cfrac{\cfrac{3}{24}}{\cfrac{3}{24}+\cfrac{5}{24}}=\cfrac{3}{8}

But for the case the person tell lies when the die is NOT rolled 6, there is only 1 5 \cfrac{1}{5} chance that the person reports the rolled number is 6 6

So the prob. should be = 3 24 3 24 + 5 24 1 5 = 3 4 = \cfrac{\cfrac{3}{24}}{\cfrac{3}{24}+\cfrac{5}{24}\cfrac{1}{5}}=\boxed{\cfrac{3}{4}}

Eric Roberts
Mar 25, 2021

I think if the probability the person tells the truth/lies is independent of the roll; Then, if said person says "its a 6", the probability of the roll being a 6 is equivalent to the probability that they tell the truth for the outcome of a given roll ( i.e it doesn't depend on how probable the roll "being a 6" is ).

Thus; P = 3 4 P = \frac{3}{4}

Your explanation is nice.

Pop Wong - 2 months, 2 weeks ago

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Thank you, I wasn't quite sure for a moment if it was faulty.

Eric Roberts - 2 months, 2 weeks ago

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