OK, let's work through this together. We're starting with 105 and we're going to multiply it by 21, which means we add 105 to itself 21 times.
Step 1: 0 + 105 = 105
Step 2: 105 + 105 = 210
Step 3: 210 + 105 = 315
Step 4: 315 + 105 = 420
Step 5: 420 + 105 = 525
Step 6: 525 + 105 = 630
Step 7: 630 + 105 = 735
Step 8: 735 + 105 = 840
Step 9: 840 + 105 = 945
Step 10: 945 + 105 = 1050
Step 11: 1050 + 105 = 1155
Step 12: 1155 + 105 = 1260
Step 13: 1260 + 105 = 1365
Step 14: 1365 + 105 = 1470
Step 15: 1470 + 105 = 1575
Step 16: 1575 + 105 = 1680
Step 17: 1680 + 105 = 1785
Step 18: 1785 + 105 = 1890
Step 19: 1890 + 105 = 1995
Step 20: 1995 + 105 = 2100
Step 21: 2100 + 105 = 2205

So, 105*21 = 2205