(i) Step 1 goes like →

12 = 22 × 31; 15 = 31 × 51;

18 = 21 × 32; 21 = 31 × 71 Step 2 goes like → 3 (only prime factor that is common across all the numbers).

Step 3 goes like → 31 (minimum power of 3 = 1). Hence, HCF = 3.

(ii) 30