2018 CSAT, all questions

UPSC Prelims 2018 · CSAT

Question 3: Logical Reasoning and Mental Ability

Consider the following pattern of numbers :

8101513
6574
4688
61116?

What is the number at ? in the above pattern?

a17
b19
c21
d23

AnswerUPSC’s answer is (a).

Look down each column. In every column, first number minus second plus third gives the fourth: 8 - 6 + 4 = 6, 10 - 5 + 6 = 11, 15 - 7 + 8 = 16. The last column therefore gives 13 - 4 + 8 = 17. In number-grid questions, test simple row and column operations first: sums, differences, and alternating add-subtract combinations of the first three entries.

Why each option is right or wrong
  • (a) Correct: 13 - 4 + 8 = 17.
  • (b) 19 does not fit first - second + third for the last column.
  • (c) 21 does not fit; the rule gives 17.
  • (d) 23 does not fit; the rule gives 17.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2018, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.