Two composite numbers do not have a common factor other than $1$.

List all the prime factors of the following numbers and find their HCF.

$20, 28$

List all the prime factors of the following numbers and find their HCF.

$12, 40$

List all the prime factors of the following numbers and find their HCF.

$30, 75$

List all the prime factors of the following numbers and find their HCF.

$72, 100$

List all the prime factors of the following numbers and find their HCF.

$63, 108$