Which one of these is Prime, 53, 65, 72, 365, 3411?

Answer 1

53
3411 is divisble by 9
the others are all clearly not prime

Answer 2

At first glance, any number that ends with an even digit is divisible by 2, so 72 is gone.

Any number that ends with a 5 or 0, is divisible by 5; 65 and 365 are gone.

Any number whose digits add up to a number divisible by 3 is itself divisible by 3:
3411: 3+4+1+1 = 9, therefore 3411 is divisible by 3.

This leaves only 53 to check:

The long but sure way is to check every prime from 7 (we've already checked 2, 3 and 5) to the square root of 53 (which is less than 8). So we only have to check 7.
Is 53 divisible by 7? no, therefore it is prime.

Answer 3

5 x 13 = 65, so 65 is not prime. (hint, anything that ends in a 5 is not going to be prime)
8 x 9 = 72, so 72 is not prime. (hint, anything other than "2" that ends in 2 is not going to be prime)
73 x 5 = 365, so 365 is not prime -- besides, it ends in 5.
1137 x 3 = 3411, so it is not prime (hint, anything that adds up to something divisible by three is itself divisible by three, and so isn't prime. 3411 -- 3 + 4 + 1 + 1 adds up to 9, which is 3 x 3.)

the only one left is 53. It isn't even, doesn't end in 5, and doesn't add up to something divisible by 3. (Those aren't the ONLY rules, but they do all apply to all the OTHER numbers here).

53 is a prime number.

-Julie

Answer 4

The only prime in the list is 53.
65 and 365 end in 5 so they are divisible by 5.
72 is even, so it is divisible by 2.
Finally, the sum of the digits of 3411 is 9, so
3411 is divisible by 9.

Answer 5

53

Answer 6

53

Answer 7

53 is only divisible by 1 and itself. All the other numbers are divisible by other numbers besides 1 and itself...

65 is definitely divisible by 5

72 is an even number, so that means that 2 is a factor of it

365 ends in 5, so it is divisible by 5

3411 is divisible by 9 because 3+4+1+1 = 9 (you can add all the digits together, and if the added digits equal something that is divisible by 9, then the entire number is divisible by 9)

Answer 8

I know that 53 is a prime and 95% sure that 3411 is not a prime number.

Answer 9

53

Answer 10

Here's how to figure it out.
65 is a multiple of 5 because it ends in 5.
72 is a multiple of two, it ends with an even number.
365 also ends in 5.
3411- the digits add up to a multiple of 3, so you know it is also a multiple of 3.

Answer 11

53 is prime
65=5*13
72=2^3*3^2
365=5*73
3411=3^2*379