Question: what is the greatest common factor between 49 and 21?
Answer: The prime factorization of 21 is: 3 x 7.
The prime factorization of 49 is: 7 x 7.
The prime factors and multiplicities 21 and 49 have in common are: 7.
7 is the gcf of 21 and 49.
gcf(21,49) = 7.