1. As visible, 12 and 15 have only one common prime factor i.e. 3. Hence, the HCF of 12 and 15 is 3.
2. To find the HCF of 20 and 25, we will find the prime factorization of the given numbers, i.e. 20 = 2 × 2 × 5; 25 = 5 × 5. ⇒ Since 5 is the only common prime factor of 20 and 25. Hence, HCF (20, 25) = 5.
