Answer:
18
Step-by-step explanation:
The GCF is also known as the greatest common factor. Every number can be written as the product of at least 2 numbers, and almost every number has at least one factor that is a prime number (or a number that is only divisible by 1 and itself).
Let's find the prime factorization of 18 and 72; in other words, let's find what prime numbers make these numbers up.
18 = 2 * 9 = 2 * 3 * 3 ⇒ notice that while 9 isn't a prime number, 2 and 3 are
72 = 2 * 36 = 2 * 2 * 18 = 2 * 2 * 2 * 3 * 3 ⇒ again, 2 and 3 are primes
In finding the GCF, we want to find the largest value of the prime numbers that the two numbers share.
Here, we see that both 18 and 72 have at least one factor of 2 and at least two factors of 3. So, that's the greatest common factor: 2 * 3 * 3 = 18.
