UPSC Prelims 2026 · CSAT
Question 25: Basic Numeracy
The class average x in a test increases by 4 when the score of a student is rectified, whose corrected score is 100 instead of 0. Later, the score of another student was found to have been recorded as 81 in place of 56. If there are no other corrections and the final corrected average is y, then y − x is
AnswerUPSC’s answer is (b).
Correcting a score from 0 to 100 adds 100 to the total and raises the average by 4, so there are 100/4 = 25 students. The second correction replaces a wrongly recorded 81 by the true 56, lowering the total by 25, so the average falls by 25/25 = 1. Taking x as the original average, the final average is x + 4 − 1, so y − x = 3.
Why each option is right or wrong
- (a) Wrong: 2 does not match +4 then −1.
- (b) Right: +4 followed by −1 gives 3.
- (c) Wrong: 5 would need the second correction to raise the average by 1.
- (d) Wrong: 6 does not match the two changes.
Only the provisional key (27 May 2026) was found; the final key replaces it when found.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2026, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.