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Question

For two events A and B, \(P(A) = P(A|B) = 0.25\) and \(P(B|A) = 0.5\). Which of the following are correct?
I. A and B are independent.
II. \(P(A^c \cup B^c) = 0.875\)
III. \(P(A^c \cap B^c) = 0.375\)
Select the answer using the code given below.

This question was previously asked in
NDA 2 2024 GAT Question Paper (01-Sep-2024)
The correct answer is
I, II and III

Let's analyze the given information and statements one by one.

  1. Given: \(P(A) = P(A|B) = 0.25\) and \(P(B|A) = 0.5\)
  2. To check if events A and B are independent, we need to verify if \(P(A|B) = P(A)\) and \(P(B|A) = P(B)\).
    • Since \(P(A) = P(A|B) = 0.25\), this condition for independence holds.
    • We also know \(P(B|A) = 0.5\).

Let's find \(P(B)\) using the formula for conditional probability:

\(P(B|A) = \frac{P(A \cap B)}{P(A)}\)

Substitute the known values:

\(0.5 = \frac{P(A \cap B)}{0.25} \Rightarrow P(A \cap B) = 0.5 \times 0.25 = 0.125\)

Now, using the formula \(P(A \cap B) = P(A) P(B)\) for independent events:

\(P(B) = \frac{P(A \cap B)}{P(A)} = \frac{0.125}{0.25} = 0.5\)

Thus, the events A and B are independent. So, statement I is correct.

Now, let's verify statement II.

Using the formula for the complement of a union:

\(P(A^c \cup B^c) = 1 - P(A \cap B)\)

Substituting the value of \(P(A \cap B) = 0.125\):

\(P(A^c \cup B^c) = 1 - 0.125 = 0.875\)

Therefore, statement II is correct.

Lastly, let's evaluate statement III.

Using De Morgan's law: \(P(A^c \cap B^c) = 1 - P(A \cup B)\)

We can use the property of probabilities:

\(P(A \cup B) = P(A) + P(B) - P(A \cap B)\)

Substitute the known values:

\(P(A \cup B) = 0.25 + 0.5 - 0.125 = 0.625\)

Therefore:

\(P(A^c \cap B^c) = 1 - 0.625 = 0.375\)

Statement III is also correct.

Hence, the correct answer is: I, II and III.

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Important Questions from Conditional Probability

  1. A and B are two events such that A̅ and B̅ are mutually exclusive. If P(A) = 0.5 and P(B) = 0.6, then what is the value of P(A|B)?

  2. For two dependent events A and B, it is given that P(A) = 0.2 and P(B) = 0.5. If A ⊆ B, then the values of conditional probabilities P(A|B) and P(B|A) are respectively

  3. In a bulb factory, machines P, Q and R manufacture respectively 25%, 35% and 40% of the total. Of their output 5, 4 and 2 percent respectively are defective bulbs. A bulb is drawn at random and it is found to be defective. What is the probability that it was manufactured by machine Q?

  4. Let X1 and X2 be independent random variables each having geometric distribution qk p ; k = 0, 1, 2, …. Then the conditional distribution of X1 given X1 + X2 is

  5. Find the probability that they speak the same fact if A speaks truth in 60% cases and B speaks truth in 75% cases?

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