A speaks the truth 8 times out of 10 times. A die is tossed. He reports that it was 5. What is the probability that it was actually 5?
Let us assume U1, U2 and A be the events as follows:
U1 = Getting 5 on throwing a die
U2 = Getting other than 5 on throwing a die
A = Reporting 5 after throwing the die
From the problem,
⇒
⇒
⇒ P(A|U1) = P(Reporting 5 on actually getting 5 on throwing a die)
⇒ P(A|U1) = P(Telling the truth)
⇒
⇒ P(A|U2) = P(Reporting 5 but not getting 5 on throwing a die)
⇒ P(A|U2) = P(Not telling the truth)
Now we find
P(U1|A) = P(the die actually shows 5 given that man reports 5)
Using Baye’s theorem:
⇒
⇒
⇒
⇒
⇒
∴ The required probability is .