The GCF is the greatest common factor. Find the factors for 45, 90, and 75; 45: 1, 3, 5, 9, 15, 45 90: 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90 75: 1, 3, 5, 15, 25, <span>75 Now find the biggest factor they have in all, which is 15. Hope this helps. </span>