Perform the following operations: find the first six multiples of 9; find the factors of 12 and count them; find the GCF of 12 and 15; list the prime numbers from the set 2, 3, 4, 5, 6, 7, 8, 9, 10, 1

First, list the factors for each number.
Factors of 12: 1, 2, 3, 4, 6, 12
Factors of 15: 1, 3, 5, 15
The common factors are 1 and 3. The greatest common factor (GCF) is the largest of these common factors.
The GCF of 12 and 15 is $\boxed{3}$.

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.

• 2 is prime.
• 3 is prime.
• 4 is not prime ($4 = 2 \times 2$).
• 5 is prime.
• 6 is not prime ($6 = 2 \times 3$).
• 7 is prime.
• 8 is not prime ($8 = 2 \times 4$).
• 9 is not prime ($9 = 3 \times 3$).
• 10 is not prime ($10 = 2 \times 5$).
• 11 is prime.
• 12 is not prime ($12 = 2 \times 6$).
• 13 is prime.
• 14 is not prime ($14 = 2 \times 7$). The set of prime numbers is $\boxed{\{2, 3, 5, 7, 11, 13\}}$.

To factorise 25, we find its prime factors.

The factors of 25 are 1, 5, and 25. Since 25 has more than two factors (1, 5, and 25), it is a composite number.
Factorisation of 25: $\boxed{5 \times 5}$
25 is a $\boxed{\text{composite number}}$.