Let's roll up our sleeves and solve this. We have 345 and we're going to multiply it by 15, essentially adding 345 to itself 15 times.
Step 1: 0 + 345 = 345
Step 2: 345 + 345 = 690
Step 3: 690 + 345 = 1035
Step 4: 1035 + 345 = 1380
Step 5: 1380 + 345 = 1725
Step 6: 1725 + 345 = 2070
Step 7: 2070 + 345 = 2415
Step 8: 2415 + 345 = 2760
Step 9: 2760 + 345 = 3105
Step 10: 3105 + 345 = 3450
Step 11: 3450 + 345 = 3795
Step 12: 3795 + 345 = 4140
Step 13: 4140 + 345 = 4485
Step 14: 4485 + 345 = 4830
Step 15: 4830 + 345 = 5175

So, 345*15 = 5175