Let's get this math done. We have 280 and we're going to multiply it by 15. This is the same as taking 280 and adding it to itself 15 times.
Step 1: 0 + 280 = 280
Step 2: 280 + 280 = 560
Step 3: 560 + 280 = 840
Step 4: 840 + 280 = 1120
Step 5: 1120 + 280 = 1400
Step 6: 1400 + 280 = 1680
Step 7: 1680 + 280 = 1960
Step 8: 1960 + 280 = 2240
Step 9: 2240 + 280 = 2520
Step 10: 2520 + 280 = 2800
Step 11: 2800 + 280 = 3080
Step 12: 3080 + 280 = 3360
Step 13: 3360 + 280 = 3640
Step 14: 3640 + 280 = 3920
Step 15: 3920 + 280 = 4200

So, 280*15 = 4200