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CBSE Class 12 Math 2018 Solved Paper

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Question : 22 of 29
Marks: +1, -0
Suppose a girl throws a die. If she gets 1 or 2, she tosses a coin three times and notes the number of tails. If she gets 3,4,5 or 6, she tosses a coin once and notes whether a ‘head’ or ‘tail’ is obtained. If she obtained exactly one ‘tail’. What is the probability that she threw 3, 4, 5 s or 6 with the die?
Solution:  
Let E1E_1 be the event that the outcome on the die is 1 or 2
P (E1)(E_1) = 26\frac{2}{6} = 13\frac{1}{3}
let E2E_2 be the event that the outcome on the die is 3,4,5 or 6
P (E2)(E_2) = 46\frac{4}{6} = 23\frac{2}{3}
let A be the event that exactly one tail comes
So
P (AE1)\left(\frac{A}{E_1}\right) = 28\frac{2}{8} (A occurs given that E1E_1 had already occured, coin tossed thrice)
P (PE2)\left(\frac{P}{E_2}\right) = 12\frac{1}{2} (A occures given that E2E_2 had already occured, coin tossed once
By Bayes's theorem, (A had already occured, probability that E2E_2 had occured)
P (E2A)\left(\frac{E_2}{A}\right) =
P(E2)×P(AE2)P(E2)×P(AE2)+P(E1)×P(AE1)\frac{P(E_2) \times P\left(\frac{A}{E_2}\right)}{P(E_2) \times P\left(\frac{A}{E_2}\right) + P(E_1) \times P\left(\frac{A}{E_1}\right)}
= 23×1223×12+13×38\frac{\frac{2}{3} \times \frac{1}{2}}{\frac{2}{3} \times \frac{1}{2} + \frac{1}{3} \times \frac{3}{8}}
= 811\frac{8}{11}
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