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Probability question

2024 · 29 Jan · Shift 1 · Q36
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  5. /2024 · 29 Jan · Shift 1 · Q36

Probability question

2024 · 29 Jan · Shift 1 · Q36

JEE MainMathematicsProbabilityMCQ+4 / −1
A fair die is thrown until 2 appears. Then the probability, that 2 appears in even number of throws, is
  1. A
    511\frac{5}{11}115​
  2. B
    56\frac{5}{6}65​
  3. C
    16\frac{1}{6}61​
  4. D
    611\frac{6}{11}116​
View written solutionFree

Correct answer: A

  1. Let the random variable NNN denote the throw on which the first 222 appears.

  2. Since the die is fair:

    • Probability of getting a 222 on any throw is p=16p = \frac{1}{6}p=61​
    • Probability of not getting a 222 is q=56q = \frac{5}{6}q=65​
  3. We need the probability that the first 222 appears on an even-numbered throw: P(N is even)=P(N=2)+P(N=4)+P(N=6)+⋯P(N \text{ is even}) = P(N=2)+P(N=4)+P(N=6)+\cdotsP(N is even)=P(N=2)+P(N=4)+P(N=6)+⋯

  4. Using geometric distribution, P(N=n)=qn−1pP(N=n)=q^{n-1}pP(N=n)=qn−1p So for even n=2,4,6,…n=2,4,6,\dotsn=2,4,6,…, P(N is even)=qp+q3p+q5p+⋯P(N \text{ is even})=qp+q^3p+q^5p+\cdotsP(N is even)=qp+q3p+q5p+⋯

  5. Factor this as a geometric series: P(N is even)=qp(1+q2+q4+⋯ )P(N \text{ is even})=qp\left(1+q^2+q^4+\cdots\right)P(N is even)=qp(1+q2+q4+⋯) Since 1+q2+q4+⋯=11−q21+q^2+q^4+\cdots = \frac{1}{1-q^2}1+q2+q4+⋯=1−q21​ we get P(N is even)=qp1−q2P(N \text{ is even})=\frac{qp}{1-q^2}P(N is even)=1−q2qp​

  6. Substitute p=16p=\frac{1}{6}p=61​ and q=56q=\frac{5}{6}q=65​: P(N is even)=(56)(16)1−(56)2P(N \text{ is even})=\frac{\left(\frac{5}{6}\right)\left(\frac{1}{6}\right)}{1-\left(\frac{5}{6}\right)^2}P(N is even)=1−(65​)2(65​)(61​)​

  7. Simplify: 5361−2536=5361136=511\frac{\frac{5}{36}}{1-\frac{25}{36}}=\frac{\frac{5}{36}}{\frac{11}{36}}=\frac{5}{11}1−3625​365​​=3611​365​​=115​

  8. Therefore, the required probability is 511\boxed{\frac{5}{11}}115​​

  9. Comparing with the options:

    • A: 511\frac{5}{11}115​ ✅
    • B: 56\frac{5}{6}65​
    • C: 16\frac{1}{6}61​
    • D: 611\frac{6}{11}116​

Hence, the correct option is A.

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