OK, let's work through this together. We're starting with 423 and we're going to multiply it by 15, which means we add 423 to itself 15 times.
Step 1: 0 + 423 = 423
Step 2: 423 + 423 = 846
Step 3: 846 + 423 = 1269
Step 4: 1269 + 423 = 1692
Step 5: 1692 + 423 = 2115
Step 6: 2115 + 423 = 2538
Step 7: 2538 + 423 = 2961
Step 8: 2961 + 423 = 3384
Step 9: 3384 + 423 = 3807
Step 10: 3807 + 423 = 4230
Step 11: 4230 + 423 = 4653
Step 12: 4653 + 423 = 5076
Step 13: 5076 + 423 = 5499
Step 14: 5499 + 423 = 5922
Step 15: 5922 + 423 = 6345

So, 423*15 = 6345