Average of the primes between 30 and 50 — Aptitude Question
Average of the primes between 30 and 50: What is the average of all prime numbers between 30 and 50? Answer and worked solution on CodeKairo.
- Topic: Averages
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
What is the average of all prime numbers between 30 and 50?
Options
- 37.8
- 38.8
- 39.8
- 40.8
Show the answer and the worked solution
Correct answer: C. 39.8
Worked solution
- The primes between 30 and 50 are 31, 37, 41, 43 and 47.
- Sum = 199, and there are 5 of them.
- Average = 199 ÷ 5 = 39.8.
Answer: 39.8.
The idea
Primes are not evenly spaced, so the midpoint shortcut fails — the list must be written out and added. The risk here is missing 43 or wrongly including 39 or 49.
Shortcut. Measure from 40: −9, −3, +1, +3, +7 sum to −1, so the average is 40 − 1/5 = 39.8.
Common trap. "40.8" — it looks right because the primes cluster around 40, but their deviations from 40 sum to −1, so the average sits just below 40, not above.