The probability of the simultaneous occurrence of two events A and B is p. If the probability that exactly one of A, B occurs is q, then which of the following alternatives is incorrect ?
Answer & explanation
Correct answer: option 1
We have,
$P(A∩ B)= p $ and $P(A) +P(B)-2P(A ∩ B)= q$
$⇒ P(A) + P(B) -2p = q $
$⇒ P(A) + P(B) =2p + q $
$⇒ 1-P(\overline{A}) +1 - P(\overline{B})= 2p+q$
$⇒P(\overline{A})+P(\overline{B})= 2 - 2p - q$
So, alternative (b) is correct.
Now,
$\begin{Bmatrix}(A ∩B)/(A ∪B)\end{Bmatrix}=\frac{P[(A ∩B)∩(A ∪B)}{P(A ∪B)}$
$⇒ P\begin{Bmatrix}(A ∩B)/(A ∪B)\end{Bmatrix}=\frac{P(A ∩B)}{P(A ∪B)}$
$⇒ P\begin{Bmatrix}(A ∩B)/(A ∪B)\end{Bmatrix}=\frac{P(A ∩B)}{P(A)+P(B)-P(A ∩B)}$
$⇒ P\begin{Bmatrix}(A ∩B)/(A ∪B)\end{Bmatrix}=\frac{p}{2p+q-p}=\frac{p}{p+q}$
So, alternative (c) is correct.
Finally,
$P(\overline{A} ∩\overline{B}) P(\overline{A ∪B})=1- P(A ∪B)$
$⇒P(\overline{A} ∩\overline{B})= P(\overline{A ∪B})=1 -[P(A) +P(B) -P(A ∩B)]$
$⇒P(\overline{A} ∩\overline{B})= P(\overline{A ∪B})=1-[2p+q-p]=1-p-q$