To find the greatest common factor (GCF) of 28 and 72, we can use the prime factorization method. First, we list the prime factors of each number:

• 28 = 2 x 2 x 7
• 72 = 2 x 2 x 2 x 3 x 3

Next, we identify the common factors and multiply them together to get the GCF. In this case, the common factors are 2 x 2, which equals 4. Therefore, the GCF of 28 and 72 is 4.