UPSC Prelims 2020 · CSAT
Question 35: Logical Reasoning and Mental Ability
A is 16th from the left end in a row of boys and V is 18th from the right end. G is 11th from A towards the right and 3rd from V towards the right end. How many boys are there in the row?
AnswerUPSC’s answer is (b).
A is 16th from the left. G is 11th from A towards the right, so G is 16 + 11 = 27th from the left. V is 18th from the right and G is 3rd from V towards the right end, so G is 18 - 3 = 15th from the right. Row length = 27 + 15 - 1 = 41. Remember to subtract 1 because G is counted from both ends.
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) 40 undercounts by one.
- (b) Right: 27 + 15 - 1 = 41.
- (c) 42 forgets that G is counted from both ends (27 + 15).
- (d) The data is enough; G's position fixes both ends.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.