2015 CSAT, all questions
UPSC Prelims 2015 · CSAT
Question 13: Logical Reasoning and Mental Ability
What is the missing number ‘X’ of the series 7, X, 21, 31, 43 ?
a11
b12
c13
d14
AnswerUPSC’s answer is (c).
Take the differences between neighbouring terms. The known gaps are 21 to 31 = 10 and 31 to 43 = 12, so the gaps grow by 2 each step. Going back, the gap before 10 is 8 (from 13 to 21) and the one before that is 6 (from 7 to 13). So X = 7 + 6 = 13, and the series 7, 13, 21, 31, 43 has gaps 6, 8, 10, 12.
Why each option is right or wrong
- (a) Wrong: 11 gives gaps 4 and 10, breaking the rise of 2 each step.
- (b) Wrong: 12 gives gaps 5 and 9, not a rise of 2.
- (c) Right: gaps 6, 8, 10, 12.
- (d) Wrong: 14 gives gaps 7 and 7, not a rise of 2.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2015, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.