What is the highest common factor (HCF) of 15 and 21 \( ? \)

To find the highest common factor (HCF) of 15 and 21, we first list the factors of each number. The factors of 15 are 1, 3, 5, and 15, while the factors of 21 are 1, 3, 7, and 21. The highest common factor between the two sets is 3, making it the HCF of 15 and 21. Now, if you want to impress your friends or just get competitive, try using prime factorization! For 15, it’s 3 × 5, and for 21, it’s 3 × 7. The only common prime factor is 3. Easy as pie, right? When calculating the HCF, it’s crucial to ensure you list all factors accurately. A common mistake is to overlook smaller factors or rush through the division method, which can lead to incorrect results. Take your time and check those factors twice for the best outcomes!