What is the mean of all the prime numbers between 20 and 40?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 30
List the primes between the two numbers, add them and divide by how many there are.
Primes between 20 and 40: 23, 29, 31, 37
Sum \(=23+29+31+37=120\)
Mean \(=120\div4=30\)
The mean is 30.
Why the other options are incorrect
Option (1) 31.8 — Incorrect. This takes 39 as a prime as well, but 39 is 3 times 13.
Option (2) 40 — Incorrect. This divides the sum 120 by 3 instead of by the 4 primes.
Option (3) 24 — Incorrect. This divides the sum 120 by 5 instead of by the 4 primes.