UPSC Prelims 2021 · CSAT
Question 80: Basic Numeracy
A man completes 7/8 of a job in 21 days. How many more days will it take him to finish the job if quantum of work is further increased by 50% ?
AnswerUPSC’s answer is (d).
In 21 days the man does 7/8 of the job, so he does 1/24 of the job per day, and the whole job takes 24 days. If the work is increased by 50%, the total becomes 1.5 jobs, or 36 days' work. He has already worked 21 days, so 36 − 21 = 15 days remain. Check: the remaining work is 1.5 − 7/8 = 5/8 of a job, and 5/8 ÷ (1/24) = 15. Remember: find the daily rate first, then scale the total work.
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) 24 is the time for the original whole job, not the extra days needed.
- (b) 21 is the time already spent, not the days remaining.
- (c) 18 days would do 3/4 of a job, more than the 5/8 left.
- (d) Right: 5/8 of a job left at 1/24 per day takes 15 days.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.