OK, let's crack this. We're given 624 and our task is to multiply it by 15. Essentially, we'll be adding 624 to itself 15 times.
Step 1: 0 + 624 = 624
Step 2: 624 + 624 = 1248
Step 3: 1248 + 624 = 1872
Step 4: 1872 + 624 = 2496
Step 5: 2496 + 624 = 3120
Step 6: 3120 + 624 = 3744
Step 7: 3744 + 624 = 4368
Step 8: 4368 + 624 = 4992
Step 9: 4992 + 624 = 5616
Step 10: 5616 + 624 = 6240
Step 11: 6240 + 624 = 6864
Step 12: 6864 + 624 = 7488
Step 13: 7488 + 624 = 8112
Step 14: 8112 + 624 = 8736
Step 15: 8736 + 624 = 9360

So, 624*15 = 9360