3 is the greatest common factor of 15 and 42 is the correct answer.
First you had to find the prime factorization of 15: 1, 3, 5, 15. And then, you had to find the prime factorization of 42: 1, 2, 3, 6, 7, 42. You can also to used their factor of 15: [tex]1*15=15, 3*5=15, 5*3=15[/tex]. And then you can also to used their factor of 42: [tex]1*42=42, 2*21=42, 3*14=42, 6*7=42, 7*6=42[/tex]. And it gave us the final answer is 3 is the greatest common factor of 15 and 42. Hope this helps! And thank you for posting your question at here on brainly, and have a great day. -Charlie
