# Aptitude - Numbers - Discussion

Discussion Forum : Numbers - General Questions (Q.No. 1)
1.
Which one of the following is not a prime number?
31
61
71
91
Explanation:
91 is divisible by 7. So, it is not a prime number.
Discussion:
105 comments Page 1 of 11.

I know that prime numbers are of the form either 6k-1 or 6K+1 . Here 91 is of the form 6k+1 but still it is prime how come?

Here we have to notice that the above mentioned is not the general form of a prime number. There is no general form for prime no's, the above mentioned 6k+1 or 6k-1 will help you to find the prime no's but not for all the values of k.

What is the meaning of " k " ? 6k+1 or 6k-1 please explain.

Tejas Mehta said:   1 decade ago
6k + 1 or 6k - 1 is a simplified form. Any prime number always satisfies this condition,where k can be 1,2,3...9.

But plz Note: Every prime no can be written in the form of 6k+1,6K-1 But every no which can be written in the form of 6k+1,6k-1 are not necessarily prime.

Here 91 can be written as 6k + 1 = 6(15) + 1 where k = 15,
But it is not prime as it is divisible by 13,7.

We can find whether a given number is prime by using the formula n*n>=p

where n is the smallest number whose square is greater then or equal to p i:e the given number.

if p is divisible by any of the prime numbers less than n then it is not a prime otherwise it is a prime number

n*n>=p plz give any example

Hiii friends .......just apply this formula to find the not prime no or prime no...
n*n>=p
1)here P is the given no
2)find the squares which is > or = to the given no P
3)find the prime nos < or = n
4)check weather the given P no is divisible by prime nos which is < or =n
5)if divisible ....it mean it have more than two factors so given P is not a prime no.
6)if not divisible it mean the given no P is prime no

Hi, santhosh.

Hi, i would like to share with a example

to find 31 prime or not

by the above formula it can be written as 6*6>=31(p)

now upto 6 the prime nos are 2,3,5,7

31 is not divisble by any of these no,

hence it would be a prime no