IDENTIFY WHETHER THE NUMBER IS PRIME OR COMPOSITE

Tell whether the number is prime or composite.

Problem 1 :

7

Solution :

The number 7 is divisible only by 1 and the number itself.

So, 7 is a prime number.

Problem 2 :

16

Solution :

By decomposing 16, we get

The factors are 1, 2, 4, 8, 16.

The number 16 has more than two factors.

So, 16 is a composite number.  

Problem 3 :

21

Solution :

By decomposing 21, we get

The factors are 1, 3, 7, 21.

The number 21 has more than two factors.

So, 21 is a composite number. 

Problem 4 :

19

Solution :

The number 19 is divisible only by 1 and the number itself.

So, 19 is a prime number.

Problem 5 :

121

Solution :

By decomposing 121, we get

The factors are 1, 11, 121.

The number 121 has more than two factors.

So, 121 is a composite number.

Problem 6 :

51

Solution :

By decomposing 51, we get

The factors are 1, 3, 17, 51.

The number 51 has more than two factors.

So, 51 is a composite number.

Problem 7 :

84

Solution :

By decomposing 81, we get

The factors are 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84.

The number 84 has more than two factors.

So, 84 is a composite number. 

Problem 8 :

141

Solution :

By decomposing 141, we get

141 does not end with 0, 2, 4, 6 or 8. So it is not divisible by 2.

Sum of the digits of 141 is 1 + 4 + 1 = 6 (divisible by 3). So, 141 is divisible by 3. Since it has more than 2 factors, it is composite numbers

The factors are 1, 3, 47, 141.