A composite number is a positive integer that has at least one positive divisor other than one or itself; in other words, it has more than two distinct positive divisors.
From the numbers you provided:
- 11 is a prime number (not composite).
- 21 is composite (divisors: 1, 3, 7, 21).
- 31 is a prime number (not composite).
- 41 is a prime number (not composite).
- 51 is composite (divisors: 1, 3, 17, 51).
Therefore, the composite numbers from your list are:
- 21
- 51