Question: what is the greatest common factor in 21 and 84?
Answer: The prime factorization of 84 is: 2 x 2 x 3 x 7. The prime factors and multiplicities 21 and 84 have in common are: 3 x 7. 3 x 7 is the gcf of 21 and 84. gcf(21,84) = 21.