UPSC Prelims 2019 · CSAT
Question 69: Basic Numeracy
Sunita cuts a sheet of paper into three pieces. Length of first piece is equal to the average of the three single digit odd prime numbers. Length of the second piece is equal to that of the first plus one-third the length of the third. The third piece is as long as the other two pieces together. The length of the original sheet of paper is
AnswerUPSC’s answer is (d).
The single-digit odd primes are 3, 5 and 7 (1 and 9 are not prime), with average 5, so the first piece is 5. Let the third piece be t. The second is 5 + t/3 and the third equals the first two together: t = 5 + 5 + t/3, so 2t/3 = 10 and t = 15. The second piece is 10. The sheet is 5 + 10 + 15 = 30 units.
Why each option is right or wrong
- (a) Wrong: pieces of 5, 10 and 15 do not add to 13.
- (b) Wrong: 15 is only the third piece.
- (c) Wrong: pieces of 5, 10 and 15 do not add to 16.
- (d) Right: 5 + 10 + 15 = 30.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2019, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.