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CBSE Class 12 Math 2012 Solved Paper

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Question : 28 of 29
Marks: +1, -0
A girl throws a die. If she get a 5 OR 6, she tosses a coin three times and notes the number of heads. If she gets 1, 2, 3 OR 4, she tosses a coin two times and notes the number of heads obtained. If she obtained exactly two heads, what is the probability that she threw 1, 2, 3 OR 4 with the die?
Solution:  
Consider the following events:
E1E_1 = Getting 5 OR 6 in a single throw of the die
E2E_2 = Getting 1, 2, 3 OR 4 in a single throw of the die
A = Getting exactly 2 heads
We have to find, P(E2E_2/A).
Since P (E2E_2/A) =
P(A∣E2)P(E2)P(A∣E1)P(E1)+P(A∣E2)P(E2)\frac{P(A|E_2)P(E_2)}{P(A|E_1)P(E_1)+P(A|E_2)P(E_2)}
Now, P(E1)P(E_1) = 26\frac{2}{6} = 13\frac{1}{3} and P(E2)P(E_2) = 46\frac{4}{6} = 23\frac{2}{3}
Also,
P (A|E1E_1) = Probability of getting exactly 2 heads when a coin is tossed 3 times = 38\frac{3}{8}
And, P (A|E2E_2) = Probability of getting 2 heads when a coin is tossed 2 times = 12\frac{1}{2}
∴ P (E2E_2|A) = 12×2338×13+12×23\frac{\frac{1}{2} \times \frac{2}{3}}{\frac{3}{8} \times \frac{1}{3} + \frac{1}{2} \times \frac{2}{3}} = 1313(38+1)\frac{\frac{1}{3}}{\frac{1}{3} \left( \frac{3}{8} + 1 \right)} = 1313(8+38)\frac{\frac{1}{3}}{\frac{1}{3} \left( \frac{8+3}{8} \right)} = 811\frac{8}{11}
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