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JEE Main20209 Jan 2020Morning ShiftMathematicsProbabilityActual

In a box, there are 20 cards, out of which 10 are labelled as A and the remaining 10 are labelled as B . Cards are drawn at random, one after the other and with replacement, till a second A card is obtained. The probability that the second A card appears before the third B card is:

Options

  1. A9 16
  2. B11 16
  3. C13 16
  4. D15 16

Correct answer

B. 11 16

Step-by-step solution

P A = 1 2   P B = 1 2 P A A + A B A + B B A + A B B A + B B A A + B A B A = 1 4 + 1 8 + 1 8 + 1 16 + 1 16 + 1 16 = 11 16

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