The greatest common divisor (GCD) of two numbers is the largest number that divides both numbers without leaving a remainder. To find the GCD of 120 and 225, we can list the factors of each number and find the largest common factor. The factors of 120 are 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, and 120. The factors of 225 are 1, 3, 5, 9, 15, 25, 45, 75, and 225. The largest common factor is 15, so the GCD of 120 and 225 is 15.