UPSC Prelims 2020 · CSAT
Question 71: Basic Numeracy
A person X can complete 20% of work in 8 days and another person Y can complete 25% of the same work in 6 days. If they work together, in how many days will 40% of the work be completed?
AnswerUPSC’s answer is (a).
X does 20% in 8 days, so 2.5% a day. Y does 25% in 6 days, so 25/6 % a day. Together they do 2.5 + 25/6 = 20/3 % a day. For 40% of the work they need 40 ÷ (20/3) = 6 days. Check: Y alone would take 9.6 days for 40%, so the pair must be faster than that.
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) Right: 40 divided by 20/3 is 6.
- (b) 8 days is what X alone needs for 20%, not what the pair needs for 40%.
- (c) 10 days is slower than Y alone would take for 40% (9.6 days).
- (d) 12 days is slower than Y alone would take for 40%.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.