OK, let's work through this together. We're starting with 123 and we're going to multiply it by 13, which means we add 123 to itself 13 times.
Step 1: 0 + 123 = 123
Step 2: 123 + 123 = 246
Step 3: 246 + 123 = 369
Step 4: 369 + 123 = 492
Step 5: 492 + 123 = 615
Step 6: 615 + 123 = 738
Step 7: 738 + 123 = 861
Step 8: 861 + 123 = 984
Step 9: 984 + 123 = 1107
Step 10: 1107 + 123 = 1230
Step 11: 1230 + 123 = 1353
Step 12: 1353 + 123 = 1476
Step 13: 1476 + 123 = 1599

So, 123*13 = 1599