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?
- (a) 45
- (b) 27
- (c) 18
- (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.