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Q: Which number is a factor of 21 and also a prime number?

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No. The only factors of a prime number are 1 and itself. The only way that 21 could be a factor of a prime number is if it were a prime number, but it is not because 21 is also divisible by 3 and 7.

They are: 3 and 7

3 is a prime factor of 12 and of 21.

If 21 is a multiple of the number, the number cannot have 90 as a factor.

3

3 and 7

21 is not a prime factor, it is 3 x 7

21 = 3 x 7 both 3 and 7 are prime numbers

7

prime factor = prime number which can be divided into the given number with no remainder The prime factors of 21 are 3 and 7.

3

3x7=21 3 and 7 are prime numbers, so 3x7 is the factor tree of the number 21.

3 x 7 = 21

Like 21? Prime numbers.

If 1 is considered a factor, the only common factor of 21 & 19 is 1, because 19 is a prime number. I'm not sure whether 1 is considered a factor since it does not reduce the original number and it's not a prime number.

the greatest common factor of 8 21 and 75 is 1.

21 = 3 x 7. Both these factors are prime.

Yes, 4 and 21 are relatively prime. Numbers that are relatively prime only have the number 1 as a common factor. That is the case with 4 and 21.

It is when the prime factors of whole number are multiplied together that they equal that number. For example the prime factors of 21 are 3 and 7 so 3*7 = 21

no

1, 3, 7, 9, 21, 63 3 and 7 are prime.

number 21 is prime

21 3,7

21 is a composite number, not a prime, having prime factors 3x7.

38 2-19= 21