UPSC Prelims 2018 · CSAT
Question 34: Basic Numeracy
A number consists of three digits of which the middle one is zero and their sum is 4. If the number formed by interchanging the first and last digits is greater than the number itself by 198, then the difference between the first and last digits is
AnswerUPSC’s answer is (b).
Let the number be a0c with a + c = 4. Interchanging the first and last digits gives c0a. The increase is (100c + a) - (100a + c) = 99(c - a) = 198, so c - a = 2. With a + c = 4, c = 3 and a = 1. The number is 103 and 301 - 103 = 198. The difference between the first and last digits is 2.
Why each option is right or wrong
- (a) A difference of 1 would raise the number by only 99.
- (b) Correct: 99 × (difference) = 198 gives a difference of 2.
- (c) A difference of 3 would raise the number by 297.
- (d) A difference of 4 would raise the number by 396.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2018, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.