2020 CSAT, all questions

UPSC Prelims 2020 · CSAT

Question 11: Basic Numeracy

In the sum ⊗ + 1⊗ + 5⊗ + ⊗⊗ + ⊗1 = 1⊗⊗ for which digit does the symbol ⊗ stand?

a2
b3
c4
d5

AnswerUPSC’s answer is (b).

Let the symbol stand for d. The sum reads d + (10 + d) + (50 + d) + 11d + (10d + 1) = 100 + 11d. The left side is 24d + 61, so 24d + 61 = 100 + 11d, which gives 13d = 39 and d = 3. Check: 3 + 13 + 53 + 33 + 31 = 133, which matches the right side with d = 3.

Why each option is right or wrong
  • (a) d = 2 gives 24 × 2 + 61 = 109 on the left but 122 on the right.
  • (b) Right: 24 × 3 + 61 = 133 = 100 + 33.
  • (c) d = 4 gives 157 on the left and 144 on the right.
  • (d) d = 5 gives 181 on the left and 155 on the right.

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

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Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.