How do you work out if 371 is a prime number?

Answer 1

By number theory, half of 371's factors will be less than its square root. Since 371 is less than 400, the square root will be less than 20. Since 371 is odd, all of its factors will be odd. By the rules of divisibility, we know that 371 is not divisible by 3, 5 or 9, so all we have to check is 7, 11, 13, 15, 17 and 19.

371 ÷ 7 = 53, so 371 is not prime.