# 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 2 of 11.

@Santhosh @Raji thanks for help.

But it only possible for some hundreds.

When we find 9999 is prime no or not,

It takes lot of time.

n*n>= p.
n*n>= 9999.
Where n must be 100.

So there are many prime no below 100.
To divided by no.

So it is not possible for all no.

Please post another method if you knew.

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

Monisha Mohanan said:   3 years ago
@All.

Prime number means that a number that is divisible only by itself and 1

Examples : 2,3,5,7,11.
Now in our question the options s 31, 61,71 and 91 here I think in our example we can find that the prime numbers only start with 2,1,3,5,7.
(44)

Sharma said:   3 years ago
91 prime or not.

Which square number is exceeds 91?

10 ^2 right. So let's take a prime number which is less than 10.
2, 3, 5, 7.

Check the given number is divisible by 2, 3, 5, 7.
If it is divisible then it is not a prime number.
If not that would be a prime number.
(28)

Renoop said:   7 years ago
2,3,5,7,11 etc .... prime no.

So
31 is not divided by 2,3,5,7,11.etc - so it is prime no.
61 is not divided by 2,3,5,7,11 -so it is prime no.
71 is not divided by 2,3,5,,7,11 so it is prime no.
91 is divided by 7 it is not prime no.

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.

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

Nagasai said:   8 years ago
If the number is 91 when we divide by 2 it is equal to 45.5 numbers in between 45.5 to 91 cannot exactly divide 91 except itself so when we take below that numbers 45.5. It is easy to tell given is prime or not.

Manju gouda said:   9 years ago
Basically a number is prime no. If it has exactly 2 distinct factors. For e.g for 13 factors are 13 and 1. So it is a prime number. But for 1 factors are 1 only. So it is not a prime number.

Hi,

11>square root of(91).
(11*11=121 =>11^2 > square root of (91)).

Then prime numbers below 11 are 2, 3, 5, 7

So 91 is divisible by 7.
Hence 91 is not a prime number.