Which one of the following numbers is not a prime number?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 391
A number is prime if no prime up to its square root divides it. Testing each option:
241 (\(\sqrt{241}\approx15.5\)): not divisible by 2, 3, 5, 7, 11 or 13, so prime.
337 (\(\sqrt{337}\approx18.4\)): not divisible by 2, 3, 5, 7, 11, 13 or 17, so prime.
391: \(391=17\times23\), so it is not prime.
571 (\(\sqrt{571}\approx23.9\)): not divisible by any prime from 2 to 23, so prime.
Tip: A number just below a perfect square may be a difference of two squares: \(391=400-9=20^2-3^2=(20-3)(20+3)=17\times23\).