2017 CSAT, all questions
UPSC Prelims 2017 · CSAT
Question 35: Basic Numeracy
There are thirteen 2-digit consecutive odd numbers. If 39 is the mean of the first five such numbers, then what is the mean of all the thirteen numbers?
a47
b49
c51
d45
AnswerUPSC’s answer is (a).
For consecutive odd numbers, the mean of the first five is the middle (third) one: 39. So the first number is 35. Thirteen consecutive odd numbers run from 35 to 35 + 2 x 12 = 59, and their mean is the middle (seventh) number: 35 + 2 x 6 = 47. In any evenly spaced list, the mean equals the middle term.
Why each option is right or wrong
- (a) Correct: the seventh number, 35 + 12 = 47, is the mean.
- (b) 49 is the eighth number, not the middle one.
- (c) 51 is the ninth number, not the middle one.
- (d) 45 is the sixth number, not the middle one.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.