Based on the provided image, it appears to be a sorting activity for categorizing numbers as Prime, Composite, or Neither. Here is the categorization of the number "21":

• Prime Numbers: Numbers greater than 1 that have no divisors other than 1 and themselves. (e.g., 17, 19)
• Composite Numbers: Numbers greater than 1 that have more than two divisors. (e.g., 16, 18, 20)
• Neither: Typically refers to 0 or 1, as they do not meet the criteria for being either prime or composite.

Number 21:

• 21 is Composite, as it has divisors other than 1 and itself (1, 3, 7, 21).

Thus, you would place 21 in the Composite category.

Do you need further explanation or details?

Here are 5 related questions to expand understanding:

1. How can we determine if a number is prime?
2. Why are 0 and 1 neither prime nor composite?
3. Can composite numbers have more than two factors? If yes, why?
4. What is the importance of prime numbers in mathematics?
5. Are all even numbers greater than 2 composite? Why?

Tip: A quick way to check if a number is composite is to see if it can be evenly divided by any prime number up to its square root.