Final answer:
The highest common factor (HCF) of 12 and 15 is 3.
Step-by-step explanation:
The highest common factor (HCF) of two numbers is the largest number that divides both of them without leaving a remainder. To find the HCF of 12 and 15, we need to determine their prime factorization.
Prime factor decomposition of 12: 12 = 2² x 3
Prime factor decomposition of 15: 15 = 3 x 5
Now, we identify the common factors between 12 and 15, which are 3. Therefore, the HCF of 12 and 15 is 3.