Question

Is any of these numbers prime and which ones: 913, 1913, 2913, 3913, 4913, 5913, 6913, 7913, 8913, 9913? List them please! Just the primes.

Answer 1

you can eliminate some which are divisible by 3
add up digits and if they come to 3,6 or 9 they are dviisible by 3

eg 2913 = 2+9+1+3 = 15; 1+5 = 6

so 2913 is not prime

Answer 2

the quickest way to check if a number is prime is by attempting to divide the number by other prime numbers starting with 2 up until you arrive at the square root of the number you are testing (rounded up).

so for 913 you try dividing it by every prime number between 2 and \(\sqrt{913}\) (which is 31, rounded up)

here's the first few prime numbers
2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71

Answer 3

and then there are indeed tricks like the one cwrw described.

Answer 4

913, 1913, 2913, 3913, 4913, 5913, 6913, 7913, 8913, 9913

after divisibility by 3 check :
913, 1913, 3913, 4913, 6913, 7913, 9913

after divisibility by 7 check :
913, 1913, 4913, 6913, 7913, 9913

after divisibility by 11 check :
1913, 4913, 6913, 7913, 9913

Answer 5

None of these remaining are easy to figure out

Answer 6

http://www.wolframalpha.com/input/?i=is+913%2C+1913%2C+2913%2C+3913%2C+4913%2C+5913%2C+6913%2C+7913%2C+8913%2C+9913+prime