OK, let's work through this together. We're starting with 387 and we're going to multiply it by 15, which means we add 387 to itself 15 times.
Step 1: 0 + 387 = 387
Step 2: 387 + 387 = 774
Step 3: 774 + 387 = 1161
Step 4: 1161 + 387 = 1548
Step 5: 1548 + 387 = 1935
Step 6: 1935 + 387 = 2322
Step 7: 2322 + 387 = 2709
Step 8: 2709 + 387 = 3096
Step 9: 3096 + 387 = 3483
Step 10: 3483 + 387 = 3870
Step 11: 3870 + 387 = 4257
Step 12: 4257 + 387 = 4644
Step 13: 4644 + 387 = 5031
Step 14: 5031 + 387 = 5418
Step 15: 5418 + 387 = 5805

So, 387*15 = 5805