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There are two persons with special traits such that :
They both meet up at a coffee shop, What is the probability that both will say "I spoke a lie yesterday"??
Quite simple puzzle but yet fun.
The probability is
$\frac{1}{7}$
For the first person :
Monday : The fact that he said a lie yesterday is wrong but he is lying so he can say "I spoke a lie yesterday"
Tuesday / Wednesday : The fact that he said a lie yesterday is true and he he lying so he can't say it
Thursday : The fact that he said a lie yesterday is true and he says the truth so he so he can say it
Friday / Saturday / Sunday : The fact that he said a lie yesterday is wrong and he says the truth so he so he can't say it
---> He can only says this sentence on Monday and Thursday
For the second person :
It's the same but with a 3 days shift so he can say this only on Thursday and Sunday.
The probability is :
$\frac{1}{7}$ which is the probability that they meet on Thursday
Note that :
this assume that they speak every day
