Reasoning : Logical Reasoning & Analytical Ability

Q 7 / 325

UPSC CSE Prelims 2026

Directions: Each item in this section contains a question followed by two statements. Answer each item using the following instructions and mark your response on the Answer Sheet accordingly.

Question: If x, y and z are integers, each greater than 1, then is x a prime number?
  1. Statement I: xy2=116
  2. Statement II: xz=261

EXPLANATION

The correct option is (a) - Select this option if the question can be answered using one of these statements alone, but cannot be answered using other statement.

[as per provisional answerkey]

Analysis

Statement I alone: xy2=116
We are given that x, y, and z are integers greater than 1. Let's find the prime factorization of 116:
116=2×58=2×2×29=29×22.
Since the equation is x×y2=116 and y must be an integer greater than 1, the only possible value for y2 is 22 (which is 4).
If y2=4, then y=2 (which is > 1).
Substituting this back: x×4=116, which gives x=29.
Since 29 is a prime number, we get a definitive "Yes" to the question "Is x a prime number?".
Statement I alone is sufficient.

Statement II alone: xz=261
We are given xz=261, where x and z are integers greater than 1. Let's find the factors of 261:
261=3×87=3×3×29=9×29.
Possible pairs for (x, z) such that both are greater than 1 are:
1. x=3, z=87 (x is prime)
2. x=9, z=29 (x is NOT prime)
3. x=29, z=9 (x is prime)
4. x=87, z=3 (x is NOT prime)
Because x can be either prime (3, 29) or composite (9, 87), we cannot determine if x is definitely a prime number.
Statement II alone is not sufficient.

Why the other options are incorrect

  • Option (b) - either statement alone: This is incorrect because Statement II fails to provide a unique answer (x could be 9 or 29), whereas Statement I provides a unique prime value for x.
  • Option (c) - both statements together: This is incorrect because Statement I is already sufficient on its own. The "together" option is only chosen when neither statement works independently.
  • Option (d) - cannot be answered: This is incorrect because Statement I provides a definitive mathematical proof that x must be 29, which is a prime number.

Key Concept

Prime factorization and integer constraints (x, y > 1) in algebraic equations to determine unique values.