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JEE Advanced 2022 Paper 2 · Mathematics · Probability · medium

Two fair dice are thrown. Let X be the event that the product of the numbers is even, Y the event that the sum is a multiple of 3, Z the event that…

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Two fair dice are thrown. Let XX be the event that the product of the numbers is even, YY the event that the sum is a multiple of 33, ZZ the event that the two numbers differ by exactly 11, and WW the event that the sum is 77. Which of the following statements is/are correct?
  1. A. XX and YY are independent
  2. B. XX and ZZ are independent
  3. C. YY and WW are independent
  4. D. XX and WW are independent
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Answer: A

Over the 3636 outcomes: P(X)=34P(X)=\frac34 (product odd only when both are odd, 99 outcomes), P(Y)=1236=13P(Y)=\frac{12}{36}=\frac13, P(Z)=1036=518P(Z)=\frac{10}{36}=\frac5{18}, P(W)=16P(W)=\frac16. Then X∩YX\cap Y: sums 3,6,9,123,6,9,12 with an even product; of the 1212 outcomes with sum divisible by 33 exactly 99 have an even product, giving P(X∩Y)=14=34⋅13P(X\cap Y)=\frac14=\frac34\cdot\frac13, so A is true. X∩ZX\cap Z: consecutive numbers always contain an even one, so P=518≠34⋅518P=\frac5{18}\ne\frac34\cdot\frac5{18}; B is false. Y∩W=∅Y\cap W=\varnothing since 77 is not a multiple of 33, but P(Y)P(W)≠0P(Y)P(W)\ne0, so C is false. X∩WX\cap W: each of the 66 outcomes with sum 77 (one odd, one even number) has an even product, so P=16≠18P=\frac16\ne\frac18; D is false.
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