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JEE Advanced 2018 Paper 2 · Mathematics · Probability · medium

A number is chosen at random from \1,2,…,60\. Let A, B, C be the events that the number is divisible by 2, by 3 and by 4 respectively.

One or more options correct

A number is chosen at random from {1,2,…,60}\{1,2,\dots,60\}. Let AA, BB, CC be the events that the number is divisible by 22, by 33 and by 44 respectively. Which of the following is (are) correct?
  1. A. AA and BB are independent events
  2. B. AA and CC are independent events
  3. C. P(A∪B∪C)=34P(A\cup B\cup C)=\dfrac34
  4. D. The events AA, BB, CC are pairwise independent
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Answer: A

Counting in {1,…,60}\{1,\dots,60\}: n(A)=30n(A)=30, n(B)=20n(B)=20, n(C)=15n(C)=15, so P(A)=12P(A)=\frac12, P(B)=13P(B)=\frac13, P(C)=14P(C)=\frac14. The number is in A∩BA\cap B exactly when it is a multiple of 66, so P(A∩B)=1060=16=P(A)P(B)P(A\cap B)=\frac{10}{60}=\frac16=P(A)P(B), and A,BA,B are independent (A true). Every multiple of 44 is even, so C⊂AC\subset A and P(A∩C)=14P(A\cap C)=\frac14, whereas P(A)P(C)=18P(A)P(C)=\frac18; hence A,CA,C are not independent (B false). Because C⊂AC\subset A, A∪B∪C=A∪BA\cup B\cup C=A\cup B and P=12+13−16=23≠34P=\frac12+\frac13-\frac16=\frac23\neq\frac34 (C false). Pairwise independence would need all three pairs to be independent, but the pair A,CA,C fails, so D is false (note B,CB,C are independent since multiples of 1212 number 5=20⋅15605=\frac{20\cdot15}{60}). Only the statement in option A is correct; a brute-force enumeration over the sixty outcomes confirms each count.
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