The greatest common factor (GCF) of 45 and 72 is: 9
Factors of 45: 1, 3, 5, 9, 15, 45
Factors of 72: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36
The common factors between the two numbers are 1, 3, and 9, but since 9 is the highest, it is considered the greatest common factor.