1999 General Studies, all questions
UPSC Prelims 1999 · General Studies
Question 131: Logical Reasoning and Mental Ability
In the sequence of numbers 5, 8, 13, X, 34, 55, 89, ……… the value of ‘X’ is
a20
b21
c23
d29
AnswerThe answer is (b).
Each term is the sum of the two before it: 5 + 8 = 13, so 8 + 13 = 21. Check with the later terms: 13 + 21 = 34, 21 + 34 = 55, 34 + 55 = 89. So X = 21. Remember: spot the pattern by testing whether a term equals the sum of the previous two.
Why each option is right or wrong
- (a) Wrong. 8 + 13 = 21, not 20.
- (b) Right. 8 + 13 = 21, and 13 + 21 = 34 fits.
- (c) Wrong. 13 + 23 = 36, not 34.
- (d) Wrong. 13 + 29 = 42, not 34.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 1999, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.