The greatest common factor (GCF) of 28 and 24 is 4. One way to find the GCF is to compare the prime factorization of each number. The prime factorization of 28 is 2 x 2 x 7, and the prime factorization of 24 is 2 x 2 x 2 x 3. The common factors are 2 x 2, which equals 4. Therefore, the GCF of 28 and 24 is 4.