The probabilities that A and B will tell the truth are 2 / 3 and 4 / 5 respectively . What is the probability that they agree with each other ?

A 3/5

B 1/3

C 2/5

D 3/4

Solution

Correct Answer: Option A

Let A be the event of A will tell truth. B be the event of B tell truth

\(P(A)=\frac23,P(A^c)=1-P(A)=\frac13\)

\(P(B)=\frac45,P(B^c)=1-P(B)=\frac15\)

When both agree then they say true or they say false together, that is 

\(A\cap B\;or\;A^c\cap B^c\)

Also these events will be mutually exclusive :

\(P(A\cap B)+P(A^c\cap B^c)=P(A)P(B)P(C)=\frac23\times\frac45+\frac13\times\frac15=\frac35\)

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