The greatest common factor (GCF) of two numbers is the largest number that divides both of them without leaving a remainder. In this case, we need to find the GCF of 45 and 60. To do this, we can list the factors of both numbers and find the largest one they have in common. The factors of 45 are 1, 3, 5, 9, 15, 45 and the factors of 60 are 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60. The largest factor they have in common is 15, therefore, 15 is the greatest common factor of 45 and 60.