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JEE Advanced 2011 · Probability (Class 12)

Let EE and FF be two independent events. The probability that exactly one of them occurs is 1125\frac{11}{25} and the probability of none of them occurring is 225\frac{2}{25}. If P(T)P(T) denotes the probability of occurrence of the event TT, then

Answers: A. P(E)=45,P(F)=35P(E)=\frac{4}{5}, P(F)=\frac{3}{5} · D. P(E)=35,P(F)=45P(E)=\frac{3}{5}, P(F)=\frac{4}{5}

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Written solution

E, F independentE,\ F \text{ independent}

P(exactly one)=1125P(\text{exactly one}) = \frac{11}{25}

P(none)=225P(\text{none}) = \frac{2}{25}

To find: P(E), P(F)\text{To find: } P(E),\ P(F)

Let p=P(E),q=P(F)\text{Let } p = P(E), \quad q = P(F)

(1−p)(1−q)=225(1-p)(1-q) = \frac{2}{25}

p(1−q)+q(1−p)=1125p(1-q) + q(1-p) = \frac{11}{25}

P(E∩F)=1−1125−225=1225P(E \cap F) = 1 - \frac{11}{25} - \frac{2}{25} = \frac{12}{25}

P(E∩F)=pq=1225P(E \cap F) = pq = \frac{12}{25}

p+q−2pq=1125p + q - 2pq = \frac{11}{25}

p+q=1125+2⋅1225=3525=75p + q = \frac{11}{25} + 2 \cdot \frac{12}{25} = \frac{35}{25} = \frac{7}{5}

t2−75 t+1225=0t^2 - \frac{7}{5}\,t + \frac{12}{25} = 0

t=75±152=45, 35t = \frac{\frac{7}{5} \pm \frac{1}{5}}{2} = \frac{4}{5},\ \frac{3}{5}

P(E)=45,P(F)=35 ⇒ option (1)P(E)=\frac{4}{5}, \quad P(F)=\frac{3}{5} \ \Rightarrow \ \text{option (1)}

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