UPSC Prelims 2019 · CSAT
Question 70: Logical Reasoning and Mental Ability
In the sequence 1, 5, 7, 3, 5, 7, 4, 3, 5, 7, how many such 5s are there which are not immediately preceded by 3 but are immediately followed by 7 ?
AnswerUPSC’s answer is (a).
List the 5s with their neighbours: the 5 in position 2 is preceded by 1 and followed by 7; the 5 in position 5 is preceded by 3 and followed by 7; the 5 in position 9 is preceded by 3 and followed by 7. Only the first 5 is not preceded by 3 while being followed by 7. So exactly one 5 qualifies.
How you could have got this
No shortcut here: in a “how many” question, ruling out options doesn’t help. Every statement has to be judged.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Right: only the first 5 qualifies.
- (b) Wrong: the other two 5s are preceded by 3.
- (c) Wrong: all three are followed by 7, but two follow a 3.
- (d) Wrong: the first 5 qualifies.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2019, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.