List the factors of 24,30, and 24. Factors of 24: 1,2,3,4,6,8,12,24. Factors of 30: 1,2,3,5,6,10,15,30. Factors of 42: 1,2,3,6,7,14,21,42. The highest number that all of these numbers is 6. Therefore, the greatest common factor of GCF of 24,30, and 42 is 6.