2015 CSAT, all questions
UPSC Prelims 2015 · CSAT
Question 31: Basic Numeracy
In a town, 45% population read magazine A, 55% read magazine B, 40% read magazine C, 30% read magazines A and B, 15% read magazines B and C, 25% read magazines A and C; and 10% read all the three magazines. What percentage do not read any magazine ?
a10%
b15%
c20%
d25%
AnswerUPSC’s answer is (c).
Use the union formula for three sets. Readers of at least one magazine = 45 + 55 + 40 − 30 − 15 − 25 + 10 = 140 − 70 + 10 = 80%. The percentage reading none = 100 − 80 = 20%. Always subtract the pair overlaps and add back the triple overlap.
Why each option is right or wrong
- (a) Wrong: 10% would need 90% reading at least one, but the formula gives 80%.
- (b) Wrong: 15% would need 85% reading at least one.
- (c) Right: 100 − 80 = 20%.
- (d) Wrong: 25% would need 75% reading at least one.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2015, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.