You can find all the prime factors of each number and then evaluate which prime factors they have in common. Take 100 and 60 for example: 100s prime factors are 2, 2, 5, 5. 60s prime factors are 2, 2, 3, 5. The common prime factors are 2, 2, 5, therefore, the greatest common factor of 100 and 60 is 20. Jan 13, 2023