UPSC Prelims 2026·CSAT·other·data sufficiency

Directions for the next 5 (five) items : 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 : X receives three coins of different denominations : 1, 2, 5, 10 and 20. If the total amount received by X is m, does X receive a coin of denomination 5? Statement I : m is not a prime number. Statement II : The sum of the digits of m is greater than 5.

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  1. ASelect this option if the question can be answered using one of these statements alone, but cannot be answered using other statementCorrect
  2. BSelect this option if the question can be answered using either statement alone
  3. CSelect this option if the question can be answered using both the statements together, but cannot be answered using either statement alone
  4. DSelect this option if the question cannot be answered even using any of the statements

Explanation

The correct answer is Option A. To determine if X receives a coin of denomination 5, we must evaluate the possible combinations of exactly three different coins chosen from the set {1, 2, 5, 10, 20}. Using the combinations formula (5C3), there are exactly 10 possible sets.

First, let us identify the sums (m) of the 4 combinations that do not contain the 5-rupee coin:

  • 1 + 2 + 10 = 13 (Sum of digits: 1+3 = 4)
  • 1 + 2 + 20 = 23 (Sum of digits: 2+3 = 5)
  • 1 + 10 + 20 = 31 (Sum of digits: 3+1 = 4)
  • 2 + 10 + 20 = 32 (Sum of digits: 3+2 = 5)

The remaining 6 sets that do contain the 5-rupee coin yield sums (m) of 8, 16, 17, 26, 27, and 35.

Evaluating Statement I: 'm is not a prime number.' The non-prime sums across all combinations are 8, 16, 26, 27, 32, and 35. This list includes m=32 (which does not contain a 5-rupee coin) and m=8 (which does). Since it yields mixed results, Statement I alone is mathematically insufficient.

Evaluating Statement II: 'The sum of the digits of m is greater than 5.' As calculated above, the combinations lacking a 5-rupee coin have digit sums of 4, 5, 4, and 5. None of these are greater than 5. Consequently, if the sum of the digits is strictly greater than 5, m must correspond to a combination that includes the 5-rupee coin (whose digit sums are 8, 7, 8, 8, 9, and 8). Statement II alone gives a definitive 'Yes' and is entirely sufficient.

Why other options are wrong:

  • Option B is incorrect because both statements are not independently sufficient; Statement I fails to give a definitive answer.
  • Option C is incorrect because the statements do not need to be combined; Statement II alone is enough.
  • Option D is incorrect because the question can indeed be answered using Statement II.

Takeaway: In CSAT Data Sufficiency logic puzzles, comprehensively listing the inverse scenarios (e.g., all sums without the 5 coin) is the fastest way to test conditional boundaries.

other: Directions for the next 5 (five) items : Each item in this section contains a question followed by two statements. Answe

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