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
Answer & explanation
Answer: (c) 50 kg
The first five and the last five together cover all nine persons, with the 5th person counted twice. So the 5th person's weight = (5 × 45 + 5 × 55) − 9 × 50 = 500 − 450 = 50 kg.
- Total of 9 persons = 9 × 50 = 450 kg.
- First 5 = 5 × 45 = 225 kg; last 5 = 5 × 55 = 275 kg.
- 225 + 275 = 500 kg, which counts the 5th person twice.
- 5th person = 500 − 450 = 50 kg.
Remember · When two overlapping groups together cover everyone, (sum of the groups) − (grand total) = the overlap.
Question and answer: UPSC's official GS Paper II (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·