UPSC Prelims 2010 · General Studies
Question 87: Basic Numeracy
Each person’s performance compared with all other persons is to be done to rank them subjectively. How many comparisons are needed in total, if there are 11 persons ?
AnswerThe answer is (b).
Each person is compared with every other person once, so the number of comparisons equals the number of pairs among 11 persons: 11 x 10 / 2 = 55. Remember: pairs from n persons = n(n - 1) / 2. The count 66 would be right for 12 persons and 45 for 10.
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.
This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.
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) 66 is the count for 12 persons.
- (b) Right: 11C2 = 55.
- (c) 54 has no basis in the count.
- (d) 45 is the count for 10 persons.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2010, General Studies, Series A. The paper on upsc.gov.in.