UPSC Prelims 2021 · CSAT
Question 28: Logical Reasoning and Mental Ability
You are given two identical sequences in two rows :
| Sequence-I : | 8 | 4 | 6 | 15 | 52·5 | 236·25 |
| Sequence-II : | 5 | A | B | C | D | E |
What is the entry in the place of C for the Sequence-II ?
AnswerUPSC’s answer is (c).
Work out the rule from Sequence-I. Each term is the previous one times a growing multiplier: 4 = 8 × 0.5, 6 = 4 × 1.5, 15 = 6 × 2.5, 52.5 = 15 × 3.5, 236.25 = 52.5 × 4.5. Apply the same multipliers to Sequence-II, starting at 5: A = 5 × 0.5 = 2.5, B = 2.5 × 1.5 = 3.75, C = 3.75 × 2.5 = 9.375. Remember: check the ratios between neighbouring terms; when they step up by a constant, the multipliers form an arithmetic progression.
Why each option is right or wrong
- (a) 2.5 is the entry A, not C.
- (b) 5 is the starting term, not C.
- (c) Right: C = 5 × 0.5 × 1.5 × 2.5 = 9.375.
- (d) 32.8125 is D (9.375 × 3.5), the term after C.
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.