2019 CSAT, all questions
UPSC Prelims 2019 · CSAT
Question 51: Logical Reasoning and Mental Ability
What is X in the sequence 4, 196, 16, 144, 36, 100, 64, X ?
a48
b64
c125
d256
AnswerUPSC’s answer is (b).
Read the terms in alternate positions. The 1st, 3rd, 5th and 7th terms are 4, 16, 36, 64, the squares of 2, 4, 6, 8. The 2nd, 4th, 6th and 8th terms are 196, 144, 100, X, the squares of 14, 12, 10 and then 8. So X = 8^2 = 64. Two interleaved series is a common pattern when the terms swing up and down.
Why each option is right or wrong
- (a) Wrong: 48 is not a perfect square.
- (b) Right: 8^2 = 64.
- (c) Wrong: 125 is not a perfect square.
- (d) Wrong: 256 is 16^2, which breaks the falling series 14, 12, 10, 8.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2019, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.