OK, let's work through this together. We're starting with 225 and we're going to multiply it by 15, which means we add 225 to itself 15 times.
Step 1: 0 + 225 = 225
Step 2: 225 + 225 = 450
Step 3: 450 + 225 = 675
Step 4: 675 + 225 = 900
Step 5: 900 + 225 = 1125
Step 6: 1125 + 225 = 1350
Step 7: 1350 + 225 = 1575
Step 8: 1575 + 225 = 1800
Step 9: 1800 + 225 = 2025
Step 10: 2025 + 225 = 2250
Step 11: 2250 + 225 = 2475
Step 12: 2475 + 225 = 2700
Step 13: 2700 + 225 = 2925
Step 14: 2925 + 225 = 3150
Step 15: 3150 + 225 = 3375

So, 225*15 = 3375