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NDA Mathematics Probability 2026 NDA 2026 (Phase 1)

NDA Mathematics Question (2026) — Solution

Question

Three events A, B and C are such that A and B are disjoint, A and C are independent, B and C are independent. If 4P(A)=2P(B)=P(C) and P(A B C)=5P(A), then what is the value of P(C) ?

Options

  1. A. 5 6
  2. B. 1 3
  3. C. 1 6
  4. D. 2 3

Answer

D. 2 3

Step-by-step solution

Let P(A) = x. From the given relation 4P(A) = 2P(B) = P(C), we have P(B) = 2x and P(C) = 4x. Since A and B are disjoint, P(A B) = 0 and P(A B C) = 0. Since A and C are independent, P(A C) = P(A)P(C) = x(4x) = 4x^2. Since B and C are independent, P(B C) = P(B)P(C) = 2x(4x) = 8x^2. Using the formula for the union of three events: P(A B C) = P(A) + P(B) + P(C) - P(A B) - P(B C) - P(C A) + P(A B C) Substituting the given values: 5x = x + 2x + 4x - 0 - 8x^2 - 4x^2 + 0 5x = 7x - 12x^2 12x^2 - 2x = 0 2x(6x - 1) = 0 Since x 0, we get x = 1 6 . Therefore, P(C) = 4x = 4 ( 1 6 ) = 2 3 . Answer: 2 3

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