UPSC Prelims 2021 · CSAT
Question 49: Logical Reasoning and Mental Ability
Consider two Statements and four Conclusions given below. You have to take the Statements to be true even if they seem to be at variance from the commonly known facts. Read all Conclusions and then decide which of the given Conclusion(s) logically follows/follow from the Statements, disregarding the commonly known facts.
- Statement-1Some greens are blues.
- Statement-2Some blues are blacks.
- Conclusion-1Some greens are blacks.
- Conclusion-2No green is black.
- Conclusion-3All greens are blacks.
- Conclusion-4All blacks are greens.
Which one of the following is correct ?
AnswerUPSC’s answer is (d).
Some greens are blues and some blues are blacks, but the greens that are blues may be different from the blues that are blacks. So greens and blacks need not overlap, which means 'some greens are blacks' (1) is not certain. They may overlap, so 'no green is black' (2) is not certain either. Nothing forces all greens to be black (3) or all blacks to be green (4). Two 'some' statements joined through a common term never give a certain conclusion, so none of the four follows. Remember: with two particular premises, no conclusion is certain.
- 1. The greens that are blue need not be the blues that are black.
- 2. Greens and blacks may overlap, so 'no green is black' is not certain.
- 3. Nothing makes every green a black.
- 4. Nothing makes every black a green.
Why each option is right or wrong
- (a) Conclusions 1 and 2 cannot both be certain; the overlap of greens and blacks is left open.
- (b) 2 and 3 are not forced; greens and blacks may or may not overlap.
- (c) 3 and 4 claim whole-set overlaps that the premises do not give.
- (d) Right: two 'some' premises give no certain link between greens and blacks.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.