To get to simplest form find the greatest common factor, or the gcf, of both numbers. To do that find all of the factors and find the one with the highest value both numbers share. So 30 would have factors of 1, 2, 3, 5, 6, 10, 15, and 30, and 42 would have the factors of 1, 2, 3, 6, 7, 14, 21, and 42. Both numbers share the factors of 1, 2, 3, and 6 so the gcf is 6. Now divide both numbers by six to get your answer.
Answer: 5/7
