UPSC Prelims 2017 · CSAT
Question 28: Basic Numeracy
Suppose the average weight of 9 persons is 50 kg. The average weight of the first 5 persons is 45 kg, whereas the average weight of the last 5 persons is 55 kg. Then the weight of the 5th person will be
AnswerUPSC’s answer is (c).
Total of 9 persons = 9 x 50 = 450 kg. The first five weigh 5 x 45 = 225 kg and the last five weigh 5 x 55 = 275 kg. Together the two groups count the 5th person twice, so 225 + 275 = 500 = 450 + (5th person). The 5th person weighs 500 - 450 = 50 kg. For overlapping groups, add the group totals, subtract the whole, and what is left is the shared member.
Why each option is right or wrong
- (a) 45 kg is the first five's average, not the 5th person's weight.
- (b) 47.5 kg does not satisfy 225 + 275 - 450.
- (c) Correct: 225 + 275 - 450 = 50.
- (d) 52.5 kg does not satisfy the equation.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.