In order to do this, you must find the least common factor shared by 42 and 49. This is 7. Then you must divide each of the numbers in question (42 and 49) to receive your new numbers, 6 and 7 (because 42 / 7 = 6 and 49 / 7 = 7). Because there are no more common factors between the two remaining numbers, 42/49 in simplest form is 6/7.
