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

Answer 1

371

It will not divide by '2' as it is not even .

It will not divide by '5' as it does not end in '5' or '0'.

It will not divide by '9' as the sum of the digits is '2'.

So try '3'

3)371 = 123.6666.... Not a whole number, so does not divide by '3'

Try '7'

7)371 = 53

No, so 371 is NOT a Prime number because it has the factors of of '7' and 53'. which are both prime numbers.

Answer 2

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.