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JEE Advanced 2021 · Probability (Class 11)

Let EE, FF and GG be three events having probabilities P(E)=18P(E)=\frac{1}{8}, P(F)=16P(F)=\frac{1}{6} and P(G)=14P(G)=\frac{1}{4}, and let P(E∩F∩G)=110P(E \cap F \cap G)=\frac{1}{10}. For any event HH, if HCH^{\text{C}} denotes its complement, then which of the following statements is(are) TRUE?

Answers: A. P(E∩F∩GC)≤140P(E \cap F \cap G^{\text{C}}) \le \frac{1}{40} · B. P(EC∩F∩G)≤115P(E^{\text{C}} \cap F \cap G) \le \frac{1}{15} · C. P(E∪F∪G)≤1324P(E \cup F \cup G) \le \frac{13}{24}

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Written solution

P(E)=18,P(F)=16,P(G)=14P(E)=\dfrac{1}{8},\quad P(F)=\dfrac{1}{6},\quad P(G)=\dfrac{1}{4}

P(E∩F∩G)=110P(E \cap F \cap G)=\dfrac{1}{10}

Check: (1) P(E∩F∩GC)≤140(2) P(EC∩F∩G)≤115\text{Check: } (1)\ P(E\cap F\cap G^{C})\le\dfrac{1}{40}\quad (2)\ P(E^{C}\cap F\cap G)\le\dfrac{1}{15}

(3) P(E∪F∪G)≤1324(4) P(EC∩FC∩GC)≤512(3)\ P(E\cup F\cup G)\le\dfrac{13}{24}\quad (4)\ P(E^{C}\cap F^{C}\cap G^{C})\le\dfrac{5}{12}

1. A⊆B⇒P(A)≤P(B)\text{1. } A \subseteq B \Rightarrow P(A) \le P(B)

2. P(E∪F∪G)≤P(E)+P(F)+P(G)\text{2. } P(E\cup F\cup G) \le P(E)+P(F)+P(G)

E∩F∩GC=(E∩F)−(E∩F∩G)E\cap F\cap G^{C} = (E\cap F) - (E\cap F\cap G)

EC∩F∩G=(F∩G)−(E∩F∩G)E^{C}\cap F\cap G = (F\cap G) - (E\cap F\cap G)

P(E∩F)≤P(E)=18P(E\cap F) \le P(E) = \dfrac{1}{8}

P(E∩F∩GC)≤18−110=140 ✓P(E\cap F\cap G^{C}) \le \dfrac{1}{8}-\dfrac{1}{10}=\dfrac{1}{40}\ \checkmark

P(F∩G)≤P(F)=16P(F\cap G) \le P(F) = \dfrac{1}{6}

P(EC∩F∩G)≤16−110=115 ✓P(E^{C}\cap F\cap G) \le \dfrac{1}{6}-\dfrac{1}{10}=\dfrac{1}{15}\ \checkmark

P(E∪F∪G)≤P(E)+P(F)+P(G)P(E\cup F\cup G) \le P(E)+P(F)+P(G)

=18+16+14=1324 ✓= \dfrac{1}{8}+\dfrac{1}{6}+\dfrac{1}{4} = \dfrac{13}{24}\ \checkmark

P(EC∩FC∩GC)=1−P(E∪F∪G)P(E^{C}\cap F^{C}\cap G^{C}) = 1 - P(E\cup F\cup G)

≥1−1324=1124\ge 1-\dfrac{13}{24} = \dfrac{11}{24}

1124>512⇒(4) is FALSE\dfrac{11}{24} > \dfrac{5}{12} \Rightarrow (4)\ \text{is FALSE}

Answer: (1), (2) and (3)\boxed{\text{Answer: (1), (2) and (3)}}

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