2024 CSAT, all questions
UPSC Prelims 2024 · CSAT
Question 79: Logical Reasoning and Mental Ability
What will come in place of ∗ in the sequence 3, 14, 39, 84, ∗, 258?
a150
b155
c160
d176
AnswerUPSC’s answer is (b).
Take the differences: 11, 25, 45, then two more. The second differences are 14, 20, so they rise by 6: 14, 20, 26, 32. The first differences therefore continue 45, 71, 103. So the missing term is 84 + 71 = 155, and 155 + 103 = 258 matches the last term. A formula that fits all six terms is n³ + n² + n.
Why each option is right or wrong
- (a) Wrong. 150 breaks the pattern of second differences rising by 6.
- (b) Right. 84 + 71 = 155, and 155 + 103 = 258.
- (c) Wrong. 160 breaks the pattern of second differences rising by 6.
- (d) Wrong. 176 breaks the pattern of second differences rising by 6.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2024, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.