UPSC Prelims 2022 CSAT Paper II · Q66 of 78 Basic Numeracy medium

Let p be a two-digit number and q be the number consisting of same digits written in reverse order. If p × q = 2430, then what is the difference between p and q?

  1. (a) 45
  2. (b) 27
  3. (c) 18
  4. (d) 9 ✓ UPSC's answer

Why the answer is (d)

• Let the two-digit number p be represented as 10a + b, where a is the tens digit and b is the units digit.

• The reversed number q is then 10b + a.

• The product p × q is given as 2430, so (10a + b)(10b + a) = 2430.

• Expanding the equation gives 100ab + 10a² + 10b² + ab = 2430, which simplifies to 101ab + 10(a² + b²) = 2430.

• Testing integer values for a and b (digits 1-9), the pair a=3 and b=6 satisfies the equation: (36)(63) = 2268 (incorrect, re-evaluating).

• Correct factorization of 2430: 2430 = 2 × 3^5 × 5. The two-digit factors are 30 and 81 (no, digits not same), 45 and 54 (digits 4,5 and 5,4). Let's check 45 × 54. 45 × 54 = 2430.

• Thus, p = 45 and q = 54 (or vice versa). The difference |p - q| = |45 - 54| = 9.

Why the other options are wrong

(a) 45
The difference between 45 and 54 is 9, not 45.
(b) 27
The difference between 45 and 54 is 9, not 27.
(c) 18
The difference between 45 and 54 is 9, not 18.

Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2022, held on 5 June 2022. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.

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