2018 CSAT, all questions

UPSC Prelims 2018 · CSAT

Question 2: Basic Numeracy

Consider the following sum : •+1•+2•+•3+•1=21•

In the above sum, • stands for

a4
b5
c6
d8

AnswerUPSC’s answer is (d).

Treat every dot as the same digit d, with 1d, 2d, d3 and d1 as two-digit numbers. Left side: d + (10 + d) + (20 + d) + (10d + 3) + (10d + 1) = 23d + 34. Right side: the number 21d = 210 + d. Setting them equal: 23d + 34 = 210 + d, so 22d = 176 and d = 8. Check: 8 + 18 + 28 + 83 + 81 = 218, which is the right side with d = 8.

Why each option is right or wrong
  • (a) With 4 the left side is 126 but the right side is 214.
  • (b) With 5 the left side is 149 but the right side is 215.
  • (c) With 6 the left side is 172 but the right side is 216.
  • (d) Correct: both sides equal 218.

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.