Sure thing! We've got 210 and we're gonna multiply it by 20. That's the same as adding 210 to itself 20 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
Step 16: 3150 + 210 = 3360
Step 17: 3360 + 210 = 3570
Step 18: 3570 + 210 = 3780
Step 19: 3780 + 210 = 3990
Step 20: 3990 + 210 = 4200

So, 210*20 = 4200