The greatest common factor (GCF) of 48 and 80 is 16. One way to find the GCF is to compare the prime factorization of each number. The prime factorization of 48 is 2 x 2 x 2 x 2 x 3, and the prime factorization of 80 is 2 x 2 x 2 x 2 x 5. The common prime factors are 2 x 2 x 2 x 2, which equals 16. Therefore, the GCF of 48 and 80 is 16.