UPSC Prelims 2024 CSAT Paper II · Q49 of 79 Mental Ability medium

Three numbers x, y, z are selected from the set of the first seven natural numbers such that x > 2y > 3z. How many such distinct triplets (x, y, z) are possible?

  1. (a) One triplet
  2. (b) Two triplets
  3. (c) Three triplets
  4. (d) Four triplets ✓ UPSC's answer

Why the answer is (d)

• The set of the first seven natural numbers is {1, 2, 3, 4, 5, 6, 7}.

• The condition is x > 2y > 3z, which implies x > 2y and 2y > 3z.

• Since z is a natural number, the minimum value for z is 1. If z = 1, then 3z = 3, so 2y must be greater than 3. The smallest integer y satisfying 2y > 3 is y = 2 (since 2*2 = 4 > 3).

• With y = 2, 2y = 4. The condition x > 4 requires x to be 5, 6, or 7. This gives three triplets: (5, 2, 1), (6, 2, 1), and (7, 2, 1).

• If z = 2, then 3z = 6, so 2y must be greater than 6. The smallest integer y satisfying 2y > 6 is y = 4 (since 2*4 = 8 > 6).

• With y = 4, 2y = 8. The condition x > 8 requires x to be greater than 8. However, the maximum number in the set is 7, so no such x exists for z = 2 or higher.

• Therefore, there are exactly four distinct triplets: (5, 2, 1), (6, 2, 1), (7, 2, 1), and (8, 4, 2) is invalid, wait, let me re-evaluate y=3. If y=3, 2y=6. 6 > 3z implies 3z < 6, so z can be 1. If z=1, 3z=3, 2y=6 > 3 is true. Then x > 6, so x can be 7. Triplet (7, 3, 1). Let's list all systematically.

• Case z=1: 3z=3. 2y > 3 => y >= 2.

• If y=2, 2y=4. x > 4 => x in {5, 6, 7}. Triplets: (5,2,1), (6,2,1), (7,2,1).

• If y=3, 2y=6. x > 6 => x in {7}. Triplet: (7,3,1).

• If y=4, 2y=8. x > 8 => No x in set.

• Case z=2: 3z=6. 2y > 6 => y >= 4.

• If y=4, 2y=8. x > 8 => No x in set.

• Total triplets are (5,2,1), (6,2,1), (7,2,1), (7,3,1). There are 4 triplets.

Why the other options are wrong

(a) One triplet
There are four valid triplets, not one.
(b) Two triplets
There are four valid triplets, not two.
(c) Three triplets
There are four valid triplets, not three.

Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2024, held on 16 June 2024. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.

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