Is 21 is a composite number or not ?
- As we already know that, the number having factors 1 and the number itself is the prime number.
- And numbers having more than these two factors are the composite numbers.
- To check whether the number 21 is composite or not first we have to find its factors.
Contents
Factors of 21:
- If we have taken numbers from 1, 2, 3…for checking factors of 21, we found that 21 has factors 1, 3, 7 and 21. Hence, we must say that 21 is a composite number.
- Thus, 21 is the composite number.
- If we multiply 21 by 1, 2, 3, then we get the multiples of 21 which are 21, 42…and so on.
About the number 21:
- 21 has more than two factors which are 1, 3, 7 and 21 and hence it is the composite number.
- 21 is the odd composite number and it is not the perfect square also.
- If we divide 21 by 1, 3, 7 and 21 then we get remainder as zero. Hence, 1, 3, 7 and 21 are the factors of 21.
Note:
- 21 is not the perfect square.
- Factors of 21: 1, 3, 7, 21
- Prime factors of 21: 3 and 7
Conclusion:
- 21 is the composite number which has factors 1, 3, 7 and 21.
- And hence, 21 is not the prime number.
Multiple Choice Questions:
1) 21 is a
a0 odd number
b) Prime number
c) Composite number
d) Both a and c
Ans: d) both a and c
2) The prime factors of a composite number 21 are
a) 1
b) 3 and 7
c) Both a and b
d) 211
Ans: b) 3 and 7
3) 1 is the not prime factor of 21 because
a) It is the prime number
b) 1 is neither prime nor composite number
c) Both a and b
d) Given statement is false
Ans: b) 1 is neither prime nor composite number