The greatest common factor (GCF) of 18 and 42 is 6. There are different methods to find the GCF of two numbers, including listing common factors, prime factorization, and long division. One way to find the GCF of 18 and 42 is to list their factors and find the greatest common factor that divides both numbers without any remainder. The factors of 18 are 1, 2, 3, 6, 9, and 18, while the factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42. The common factors of 18 and 42 are 1, 2, 3, and 6, and the greatest common factor is 6. Another way to find the GCF of 18 and 42 is to use prime factorization. The prime factorization of 18 is 2 x 3 x 3, while the prime factorization of 42 is 2 x 3 x 7. The common prime factors of 18 and 42 are 2 and 3, so the GCF is 2 x 3 = 6. Long division is another method to find the GCF of two numbers, but it is more complicated and less commonly used.