Question: what is highest common factor of 21 and 54?
Answer: The prime factorization of 21 is: 3 x 7.
The prime factorization of 54 is: 2 x 3 x 3 x 3.
The prime factors and multiplicities 21 and 54 have in common are: 3.
3 is the gcf of 21 and 54.
gcf(21,54) = 3.