UPSC Prelims 2014 · CSAT
Question 67: Logical Reasoning and Mental Ability
In a row 'A' is in the 11th position from the left and 'B' is in the 10th position from the right. If 'A' and 'B' interchange, then 'A' becomes 18th from the left. How many persons are there in the row other than 'A' and 'B' ?
AnswerUPSC’s answer is (c).
A is 11th from the left and B is 10th from the right. After they swap, A takes B's old place and becomes 18th from the left, so B was 18th from the left. B is also 10th from the right, so the row has 18 + 10 - 1 = 27 people. Excluding A and B, that leaves 25.
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) 27 is the whole row, including A and B.
- (b) 26 removes only one of A and B.
- (c) Right. 27 - 2 = 25.
- (d) 24 removes one person too many.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2014, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.