UPSC Prelims 2024 CSAT Paper II · Q73 of 79 Data Interpretation medium

A Question is given followed by two Statements I and II. Consider the Question and the Statements. Age of each of P and Q is less than 100 years but more than 10 years. If you interchange the digits of the age of P, the number represents the age of Q. Question : What is the difference of their ages? Statement-I : The age of P is greater than the age of Q. Statement-II : The sum of their ages is 11/6 times their difference. 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 ✓ UPSC's answer
  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
  4. (d) The Question cannot be answered even by using both the Statements together

Why the answer is (a)

• Let the age of P be $10x + y$ and the age of Q be $10y + x$, where $x$ and $y$ are digits and $x \neq y$.

• The difference in their ages is $|(10x + y) - (10y + x)| = 9|x - y|$.

• Statement I states that P is older than Q, so $10x + y > 10y + x$, which implies $x > y$. However, this only establishes the sign of the difference, not its magnitude, as $|x - y|$ can be any integer from 1 to 9.

• Statement II states that the sum of their ages is $11/6$ times their difference. The sum is $(10x + y) + (10y + x) = 11(x + y)$.

• Substituting into the equation: $11(x + y) = \frac{11}{6} \times 9|x - y|$, which simplifies to $x + y = \frac{3}{2}|x - y|$.

• Since $x$ and $y$ are integers, $|x - y|$ must be even. Let $|x - y| = 2k$. Then $x + y = 3k$. Also, $x + y$ and $x - y$ have the same parity, which is consistent. The possible values for $|x - y|$ are 2, 4, 6, or 8. This yields multiple possible differences (18, 36, 54, 72), so Statement II alone is insufficient.

• Wait, re-evaluating Statement I: Does it provide a unique value? No. Re-evaluating Statement II: Does it provide a unique value? No. Let's re-read the options and the key. The key is (a). This implies one statement is sufficient. Let's re-examine Statement I. Is there a constraint I missed? 'Age ... less than 100 ... more than 10'. This means the tens digit cannot be 0. So $x \ge 1$ and $y \ge 1$? No, if Q's age is $10y+x$, and age > 10, then $y$ cannot be 0. So $x, y \in \{1, ..., 9\}$.

• Let's re-evaluate Statement II. $x+y = 1.5 |x-y|$. If $|x-y|=2$, $x+y=3$. No integer solution for $x,y \ge 1$ with diff 2 and sum 3 (e.g., 2.5, 0.5). If $|x-y|=4$, $x+y=6$. Solutions: $x=5, y=1$ (diff 4, sum 6). Ages 51 and 15. Diff 36. If $|x-y|=6$, $x+y=9$. Solutions: $x=7.5$ (no). $x=8, y=2$ (diff 6, sum 10 - wait, $8+2=10 \ne 9$). $x=7, y=1$ (diff 6, sum 8). No. If $|x-y|=8$, $x+y=12$. Solutions: $x=10, y=2$ (x not digit). $x=9, y=1$ (diff 8, sum 10). No.

• Let's check $|x-y|=2$ again. $x+y=3$. Pairs: (2,1) sum 3, diff 1. (1,2) sum 3, diff 1. Diff is 1, not 2. So no solution for diff 2.

• Let's check $|x-y|=4$. $x+y=6$. Pairs with diff 4: (5,1) sum 6. Valid. Ages 51, 15. Diff 36. (6,2) sum 8. (7,3) sum 10. (8,4) sum 12. (9,5) sum 14. Only (5,1) works.

• Let's check $|x-y|=6$. $x+y=9$. Pairs with diff 6: (7,1) sum 8. (8,2) sum 10. (9,3) sum 12. No solution.

• Let's check $|x-y|=8$. $x+y=12$. Pairs with diff 8: (9,1) sum 10. No solution.

• So Statement II yields a UNIQUE solution: $x=5, y=1$ (or vice versa). The difference is $9 \times 4 = 36$. Thus, Statement II is sufficient.

• Statement I only says $P > Q$, so $x > y$. This allows many pairs like (2,1), (3,1), etc. It is not sufficient.

• Therefore, the question can be answered by Statement II alone, but not by Statement I alone. This matches option (a).

Why the other options are wrong

(b) The Question can be answered by using either Statement alone
Statement I alone is insufficient because it only establishes that P is older than Q without fixing the specific digit difference, allowing for multiple possible age differences.
(c) The Question can be answered by using both the Statements together, but cannot be…
The question can be answered using Statement II alone, so it is incorrect to claim that both statements are required together.
(d) The Question cannot be answered even by using both the Statements together
The question can be answered using Statement II alone, which uniquely determines the age difference as 36 years, so it is incorrect to claim it cannot be answered.

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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