UPSC Prelims 2016 · CSAT
Question 29: Basic Numeracy
In aid of charity, every student in a class contributes as many rupees as the number of students in that class. With the additional contribution of Rs. 2 by one student only, the total collection is Rs. 443. Then how many students are there in the class ?
AnswerUPSC’s answer is (b).
If the class has n students, the collection is n x n = n^2 rupees, plus Rs 2 extra: n^2 + 2 = 443, so n^2 = 441 and n = 21. Check: 21 x 21 = 441. Remember: when each of n people gives n, the total is a perfect square, so look for the square nearest the total.
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) 12 x 12 + 2 = 146.
- (b) Right: 21 x 21 + 2 = 443.
- (c) 43 x 43 + 2 = 1,851.
- (d) 45 x 45 + 2 = 2,027.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2016, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.