OK, let's crack this. We're given 210 and our task is to multiply it by 15. Essentially, we'll be adding 210 to itself 15 times.
Step 1: 0 + 210 = 210
Step 2: 210 + 210 = 420
Step 3: 420 + 210 = 630
Step 4: 630 + 210 = 840
Step 5: 840 + 210 = 1050
Step 6: 1050 + 210 = 1260
Step 7: 1260 + 210 = 1470
Step 8: 1470 + 210 = 1680
Step 9: 1680 + 210 = 1890
Step 10: 1890 + 210 = 2100
Step 11: 2100 + 210 = 2310
Step 12: 2310 + 210 = 2520
Step 13: 2520 + 210 = 2730
Step 14: 2730 + 210 = 2940
Step 15: 2940 + 210 = 3150

So, 210*15 = 3150