3 x 7
21 is not prime.
3
No; both are multiples of other numbers, including 3.
67 is a prime number, but 21 isn't. A prime number can only be evenly divided by '1' and itself. 21 can be evenly divided by 1, 3, 7, and 21.
I suggest factoring each pair of numbers, and checking whether they have, or don't have, common factors. A pair of numbers is said to be "relatively prime" if they have no common factors (their greatest common factor is 1). For larger numbers, Euclid's algorithm could be used, but for such small numbers, factoring is probably faster.
Do you mean, "What is the prime factorization of 21?"
21
21 has two prime factors.
The prime numbers from 21 to 40 are 23, 29, 31, and 37.
7 and 3 are prime numbers but 21 is a composite nuber
There are 3 prime numbers from 21 to 35:23, 29 and 31
The prime factors of 21 are 3 x 7.