JEE Challenger
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Conditional Probability of Complements of Pairwise Independent Events

Let EcE^c denote the complement of an event EE. Let E,F,GE, F, G be pairwise independent events with P(G)>0P(G) > 0 and P(E∩F∩G)=0P(E \cap F \cap G) = 0. Then P(Ec∩Fc∣G)P(E^c \cap F^c \mid G) equals

Options

A

P(Ec)+P(Fc)P(E^c) + P(F^c)

B

P(Ec)−P(Fc)P(E^c) - P(F^c)

C

P(Ec)−P(F)P(E^c) - P(F)

Correct
D

P(E)−P(Fc)P(E) - P(F^c)

Step-by-Step Solution

By De Morgan's Law, Ec∩Fc=(E∪F)cE^c \cap F^c = (E \cup F)^c. Therefore:
P(Ec∩Fc∣G)=1−P(E∪F∣G)P(E^c \cap F^c \mid G) = 1 - P(E \cup F \mid G)

By the addition theorem of probability conditioned on GG:
P(E∪F∣G)=P(E∣G)+P(F∣G)−P(E∩F∣G)P(E \cup F \mid G) = P(E \mid G) + P(F \mid G) - P(E \cap F \mid G)

  1. Since E,GE, G and F,GF, G are pairwise independent:
    P(E∣G)=P(E),P(F∣G)=P(F)P(E \mid G) = P(E), \quad P(F \mid G) = P(F)

  2. The conditional joint probability is:
    P(E∩F∣G)=P(E∩F∩G)P(G)=0P(G)=0P(E \cap F \mid G) = \frac{P(E \cap F \cap G)}{P(G)} = \frac{0}{P(G)} = 0

Substituting these back:
P(E∪F∣G)=P(E)+P(F)−0=P(E)+P(F)P(E \cup F \mid G) = P(E) + P(F) - 0 = P(E) + P(F)

Therefore:
P(Ec∩Fc∣G)=1−[P(E)+P(F)]=[1−P(E)]−P(F)=P(Ec)−P(F)P(E^c \cap F^c \mid G) = 1 - [P(E) + P(F)] = [1 - P(E)] - P(F) = P(E^c) - P(F)

Correct Answer: C