To determine if 21 is a prime number or a composite number, we simply look at the factors for 21.
If the factors of 21 are only 21 and 1, then we know that 21 is a prime number. Otherwise, 21 is a composite number.
To explain better, if 21 is only divisible by 1 and 21, then it is a prime number, and if 21 is divisible by more than 21 and 1, then it is a composite number.
The factors of 21 are as follows:
1, 3, 7, 21
Therefore, since 21 is divisible by more than one and itself, the answer to the
question "Is 21 prime or composite?" is highlighted in blue below.
composite
Is 22 prime or composite?
OK. Now you know that 21 is a composite number. Use our logic above and see if
you can figure out if the next number on our list is prime or composite before you check your answer.
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