One way to find the GCF of 90 and 100 is to compare the prime factorization of each number. To find the prime factorization, you can follow the instructions for each number here:
Here is the prime factorization of 90:
And this is the prime factorization of 100:
When you compare the prime factorization of these two numbers, you can see that there are matching prime factors. You can now find the Greatest Common Factor of 90 and 100 by multiplying all the matching prime factors to get a GCF of 90 and 100 as 100:
Thus, the GCF of 90 and 100 is: 100