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JEE Advanced 2020 Paper 2 · Mathematics · Probability · medium

A fair die is rolled three times. Let X and Y denote the largest and the smallest of the three numbers obtained.

One or more options correct

A fair die is rolled three times. Let XX and YY denote the largest and the smallest of the three numbers obtained. Which of the following statements is/are correct?
  1. A. P(X=4)=37216P(X=4)=\dfrac{37}{216}
  2. B. P(X≥5)=1927P(X\ge5)=\dfrac{19}{27}
  3. C. Given that the three numbers are all different, the probability that X=5X=5 is 310\dfrac{3}{10}
  4. D. P(Y=2)=16P(Y=2)=\dfrac16
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Answer: A, B, C

There are 216216 equally likely outcomes. (A) P(X=4)=43−33216=37216P(X=4)=\frac{4^3-3^3}{216}=\frac{37}{216}, true. (B) P(X≥5)=1−P(X≤4)=1−64216=152216=1927P(X\ge5)=1-P(X\le4)=1-\frac{64}{216}=\frac{152}{216}=\frac{19}{27}, true. (C) Conditioned on all three numbers being different there are 6⋅5⋅4=1206\cdot5\cdot4=120 outcomes; those with maximum 55 use 55 together with two distinct numbers from {1,2,3,4}\{1,2,3,4\}, giving (42)⋅3!=36\binom42\cdot3!=36 outcomes, so the probability is 36120=310\frac{36}{120}=\frac3{10}, true. (D) P(Y=2)=53−43216=61216P(Y=2)=\frac{5^3-4^3}{216}=\frac{61}{216}, which is not 16=36216\frac16=\frac{36}{216}, so false. All three true statements were also confirmed by listing the 216216 triples. Each of the four statements is therefore decided independently by counting favourable triples among the 216, and the keyed options A, B and C are exactly the true ones, with D failing because 61216≠16\frac{61}{216}\ne\frac16.
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