UPSC Prelims 2022 · CSAT
Question 28: Basic Numeracy
A has some coins. He gives half of the coins and 2 more to B. B gives half of the coins and 2 more to C. C gives half of the coins and 2 more to D. The number of coins D has now, is the smallest two-digit number. How many coins does A have in the beginning?
AnswerUPSC’s answer is (d).
Work backwards. D holds the smallest two-digit number, 10, all received from C: half of C's coins plus 2 equals 10, so C had 16. C's 16 came from B: half plus 2 equals 16, so B had 28. B's 28 came from A: half plus 2 equals 28, so A had 52. Check: A gives 28 and keeps 24; B gives 16 and keeps 12; C gives 10 and keeps 6.
How you could have got this
No shortcut here: in a “how many” question, ruling out options doesn’t help. Every statement has to be judged.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) 76 leaves D with 13 coins.
- (b) 68 leaves D with 12 coins.
- (c) 60 leaves D with 11 coins.
- (d) Correct: 52 leaves D with 10 coins.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2022, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.