The greatest common factor (GCF) of 15 and 45 can be found by identifying the factors of each number.
The factors of 15 are: 1, 3, 5, 15
The factors of 45 are: 1, 3, 5, 9, 15, 45
The common factors of 15 and 45 are: 1, 3, 5, 15
The greatest of these common factors is 15.
Therefore, the greatest common factor of 15 and 45 is 15.