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Probability question

2016 · Shift 0 · Q24
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Probability question

2016 · Shift 0 · Q24

JEE MainMathematicsProbabilityMCQ+4 / −1
Let two fair six-faced dice AAA and BBB be thrown simultaneously. If E1{E_1}E1​ is the event that die AAA shows up four, E2{E_2}E2​ is the event that die BBB shows up two and E3{E_3}E3​ is the event that the sum of numbers on both dice is odd, then which of the following statements is NOTNOTNOT true?
  1. A
    E1{E_1}E1​ and E2{E_2}E2​ are independent.
  2. B
    E2{E_2}E2​ and E3{E_3}E3​ are independent.
  3. C
    E1{E_1}E1​ and E3{E_3}E3​ are independent.
  4. D
    E1,E2{E_1},{E_2}E1​,E2​ and E3{E_3}E3​ are independent.
View written solutionFree

Correct answer: D

  1. Sample space and basic probabilities

When two fair six-faced dice AAA and BBB are thrown simultaneously, the total number of equally likely outcomes is 6×6=36.6\times 6=36.6×6=36.

We define:

  • E1E_1E1​: die AAA shows 444
  • E2E_2E2​: die BBB shows 222
  • E3E_3E3​: sum on the two dice is odd

So, P(E1)=636=16,P(E_1)=\frac{6}{36}=\frac{1}{6},P(E1​)=366​=61​, because if die A=4A=4A=4, die BBB can be anything.

Similarly, P(E2)=636=16.P(E_2)=\frac{6}{36}=\frac{1}{6}.P(E2​)=366​=61​.

Now for E3E_3E3​: The sum is odd when one die is odd and the other is even. Each die has 333 odd and 333 even faces, so P(E_3)=\frac{3\cdot 3+3\cdot 3}{36}= rac{18}{36}= rac{1}{2}.


  1. Check option A: E1E_1E1​ and E2E_2E2​ are independent

We compute: P(E1∩E2)=136,P(E_1\cap E_2)=\frac{1}{36},P(E1​∩E2​)=361​, since this is the single outcome (4,2)(4,2)(4,2).

Also, P(E1)P(E2)=16⋅16=136.P(E_1)P(E_2)=\frac{1}{6}\cdot \frac{1}{6}=\frac{1}{36}.P(E1​)P(E2​)=61​⋅61​=361​.

Thus, P(E1∩E2)=P(E1)P(E2),P(E_1\cap E_2)=P(E_1)P(E_2),P(E1​∩E2​)=P(E1​)P(E2​), so E1E_1E1​ and E2E_2E2​ are independent.

Hence, A is true.


  1. Check option B: E2E_2E2​ and E3E_3E3​ are independent

If E2E_2E2​ occurs, then die B=2B=2B=2 (which is even). For the sum to be odd, die AAA must be odd: 1,3,51,3,51,3,5. So favorable outcomes are: (1,2),(3,2),(5,2),(1,2),(3,2),(5,2),(1,2),(3,2),(5,2), which are 333 outcomes.

Therefore, P(E2∩E3)=336=112.P(E_2\cap E_3)=\frac{3}{36}=\frac{1}{12}.P(E2​∩E3​)=363​=121​.

Now, P(E2)P(E3)=16⋅12=112.P(E_2)P(E_3)=\frac{1}{6}\cdot \frac{1}{2}=\frac{1}{12}.P(E2​)P(E3​)=61​⋅21​=121​.

Thus, P(E2∩E3)=P(E2)P(E3),P(E_2\cap E_3)=P(E_2)P(E_3),P(E2​∩E3​)=P(E2​)P(E3​), so E2E_2E2​ and E3E_3E3​ are independent.

Hence, B is true.


  1. Check option C: E1E_1E1​ and E3E_3E3​ are independent

If E1E_1E1​ occurs, then die A=4A=4A=4 (which is even). For the sum to be odd, die BBB must be odd: 1,3,51,3,51,3,5. So favorable outcomes are: (4,1),(4,3),(4,5),(4,1),(4,3),(4,5),(4,1),(4,3),(4,5), which are again 333 outcomes.

Therefore, P(E1∩E3)=336=112.P(E_1\cap E_3)=\frac{3}{36}=\frac{1}{12}.P(E1​∩E3​)=363​=121​.

And, P(E1)P(E3)=16⋅12=112.P(E_1)P(E_3)=\frac{1}{6}\cdot \frac{1}{2}=\frac{1}{12}.P(E1​)P(E3​)=61​⋅21​=121​.

Thus, P(E1∩E3)=P(E1)P(E3),P(E_1\cap E_3)=P(E_1)P(E_3),P(E1​∩E3​)=P(E1​)P(E3​), so E1E_1E1​ and E3E_3E3​ are independent.

Hence, C is true.


  1. Check option D: E1,E2,E3E_1,E_2,E_3E1​,E2​,E3​ are independent

For three events to be independent, we need:

  • pairwise independence, and

P(E1∩E2∩E3)=P(E1)P(E2)P(E3).P(E_1\cap E_2\cap E_3)=P(E_1)P(E_2)P(E_3).P(E1​∩E2​∩E3​)=P(E1​)P(E2​)P(E3​).

We already found pairwise independence above.

Now check the triple intersection:

  • E1E_1E1​ means A=4A=4A=4
  • E2E_2E2​ means B=2B=2B=2 So together this gives the outcome (4,2)(4,2)(4,2) only. But then the sum is 4+2=6,4+2=6,4+2=6, which is even, not odd. Hence, E1∩E2∩E3=∅,E_1\cap E_2\cap E_3=\varnothing,E1​∩E2​∩E3​=∅, and therefore P(E1∩E2∩E3)=0.P(E_1\cap E_2\cap E_3)=0.P(E1​∩E2​∩E3​)=0.

But P(E1)P(E2)P(E3)=16⋅16⋅12=172≠0.P(E_1)P(E_2)P(E_3)=\frac{1}{6}\cdot \frac{1}{6}\cdot \frac{1}{2}=\frac{1}{72}\neq 0.P(E1​)P(E2​)P(E3​)=61​⋅61​⋅21​=721​=0.

So the three events are not mutually independent.

Hence, D is false.


  1. Conclusion

The statement which is NOT true is: D\boxed{\text{D}}D​

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