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UPSC CSE 2024 · PYQ · Data Sufficiency / Number Properties · hard

A Question is given followed by two Statements I and II. Consider the Question and the Statements. Question: Is (x + y) an integer? Statement-I: (2x + y) is an integer. Statement-II: (x + 2y) is an integer. Which one of the following is correct in respect of the above Question and the Statements?

  1. A.The Question can be answered by using one of the Statements alone, but cannot be answered using the other Statement alone
  2. B.The Question can be answered by using either Statement alone
  3. C.The Question can be answered by using both the Statements together, but cannot be answered using either Statement alone✓ Correct
  4. D.The Question cannot be answered even by using both the Statements together

Explanation

Statement I alone: 2x + y is an integer. This doesn't tell whether x + y is integer (e.g., x = 0.5, y = 0.5 gives 2x+y = 1.5 not integer; try x = 0.5, y = 0 gives 2x+y = 1 integer but x+y = 0.5 not integer). So Statement I alone insufficient. Statement II alone: x + 2y is an integer. Similarly insufficient (x = 0, y = 0.5 gives x+2y = 1 integer, x+y = 0.5 not integer). Combining: adding the two integers gives (2x+y) + (x+2y) = 3x + 3y = 3(x+y), which is an integer. So 3(x+y) is an integer, meaning x+y could be of the form k/3 (rational with denominator dividing 3). For example x = 1/3, y = 1/3 gives 2x+y = 1 integer ✓, x+2y = 1 integer ✓, but x+y = 2/3 not integer. So even both together do not guarantee x+y is integer. Hence the answer is (d), the question cannot be answered even using both statements together.
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