Class CBSE Class 12 Mathematics Probability Q #1325
COMPETENCY BASED
UNDERSTAND
3 Marks 2024 AISSCE(Board Exam) SA
A biased die is twice as likely to show an even number as an odd number. If such a die is thrown twice, find the probability distribution of the number of sixes. Also, find the mean of the distribution.

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Detailed Solution

Step 1: Determine the probabilities of even and odd numbers.

Let $P(\text{odd}) = p$. Then $P(\text{even}) = 2p$. Since the sum of probabilities must be 1, we have: $P(\text{odd}) + P(\text{even}) = 1$ $p + 2p = 1$ $3p = 1$ $p = \frac{1}{3}$ So, $P(\text{odd}) = \frac{1}{3}$ and $P(\text{even}) = \frac{2}{3}$.

Step 2: Calculate the probability of rolling a six.

Since 6 is an even number, the probability of rolling a 6 is $\frac{2}{3}$ divided by the number of even numbers (2, 4, 6), which is 3. Thus, $P(6) = \frac{P(\text{even})}{3} = \frac{2/3}{3} = \frac{2}{9}$ The probability of not rolling a 6 is $P(\text{not 6}) = 1 - P(6) = 1 - \frac{2}{9} = \frac{7}{9}$.

Step 3: Define the random variable and its possible values.

Let $X$ be the number of sixes in two throws. $X$ can take values 0, 1, or 2.

Step 4: Calculate the probabilities for each value of X.

We use the binomial probability formula: $P(X=k) = \binom{n}{k} p^k (1-p)^{n-k}$, where $n=2$ and $p = \frac{2}{9}$. $P(X=0) = \binom{2}{0} (\frac{2}{9})^0 (\frac{7}{9})^2 = 1 \cdot 1 \cdot \frac{49}{81} = \frac{49}{81}$ $P(X=1) = \binom{2}{1} (\frac{2}{9})^1 (\frac{7}{9})^1 = 2 \cdot \frac{2}{9} \cdot \frac{7}{9} = \frac{28}{81}$ $P(X=2) = \binom{2}{2} (\frac{2}{9})^2 (\frac{7}{9})^0 = 1 \cdot \frac{4}{81} \cdot 1 = \frac{4}{81}$

Step 5: Form the probability distribution.

The probability distribution is: $X = 0: P(X=0) = \frac{49}{81}$ $X = 1: P(X=1) = \frac{28}{81}$ $X = 2: P(X=2) = \frac{4}{81}$

Step 6: Calculate the mean of the distribution.

The mean $\mu$ is given by $\mu = \sum x_i P(X=x_i)$. $\mu = 0 \cdot \frac{49}{81} + 1 \cdot \frac{28}{81} + 2 \cdot \frac{4}{81} = 0 + \frac{28}{81} + \frac{8}{81} = \frac{36}{81} = \frac{4}{9}$

Final Answer: Probability Distribution: P(X=0) = 49/81, P(X=1) = 28/81, P(X=2) = 4/81. Mean = 4/9

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Pedagogical Audit
Bloom's Analysis: This is an UNDERSTAND question because the student needs to understand the concept of probability, probability distribution, and how to calculate the mean of a probability distribution. They also need to understand how the bias in the die affects the probabilities.
Knowledge Dimension: CONCEPTUAL
Justification: The question requires understanding of probability distributions, expected value, and how to apply these concepts to a biased die. It's not just about recalling facts but understanding the underlying principles.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. The question requires the student to apply their knowledge of probability to a real-world scenario (a biased die) and calculate the probability distribution and mean. This goes beyond simply recalling formulas and requires problem-solving skills.