The GCF of 75 and 90 is 15. To calculate the greatest common factor (GCF) of 75 and 90, we need to factor each number (factors of 75 = 1, 3, 5, 15, 25, 75; factors of 90 = 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90) and choose the greatest factor that exactly divides both 75 and 90, i.e., 15.