OK, let's work through this together. We're starting with 321 and we're going to multiply it by 15, which means we add 321 to itself 15 times.
Step 1: 0 + 321 = 321
Step 2: 321 + 321 = 642
Step 3: 642 + 321 = 963
Step 4: 963 + 321 = 1284
Step 5: 1284 + 321 = 1605
Step 6: 1605 + 321 = 1926
Step 7: 1926 + 321 = 2247
Step 8: 2247 + 321 = 2568
Step 9: 2568 + 321 = 2889
Step 10: 2889 + 321 = 3210
Step 11: 3210 + 321 = 3531
Step 12: 3531 + 321 = 3852
Step 13: 3852 + 321 = 4173
Step 14: 4173 + 321 = 4494
Step 15: 4494 + 321 = 4815

So, 321*15 = 4815