2021 CSAT, all questions
UPSC Prelims 2021 · CSAT
Question 50: Logical Reasoning and Mental Ability
What is the value of 'X' in the sequence 2, 7, 22, 67, 202, X, 1822 ?
a603
b605
c607
d608
AnswerUPSC’s answer is (c).
Look for the step between terms. 2 × 3 + 1 = 7, 7 × 3 + 1 = 22, 22 × 3 + 1 = 67 and 67 × 3 + 1 = 202. So each term is three times the previous plus one. X = 202 × 3 + 1 = 607, and 607 × 3 + 1 = 1822, which matches the given last term. Remember: when terms grow roughly threefold, test 'times 3 plus or minus a constant', then verify against the term after the gap.
Why each option is right or wrong
- (a) 603 would make the next term 603 × 3 + 1 = 1810, not 1822.
- (b) 605 would make the next term 1816, not 1822.
- (c) Right: 202 × 3 + 1 = 607 and 607 × 3 + 1 = 1822.
- (d) 608 would make the next term 1825, not 1822.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.