Sure thing! We've got 20 and we're gonna multiply it by 21. That's the same as adding 20 to itself 21 times.
Step 1: 0 + 20 = 20
Step 2: 20 + 20 = 40
Step 3: 40 + 20 = 60
Step 4: 60 + 20 = 80
Step 5: 80 + 20 = 100
Step 6: 100 + 20 = 120
Step 7: 120 + 20 = 140
Step 8: 140 + 20 = 160
Step 9: 160 + 20 = 180
Step 10: 180 + 20 = 200
Step 11: 200 + 20 = 220
Step 12: 220 + 20 = 240
Step 13: 240 + 20 = 260
Step 14: 260 + 20 = 280
Step 15: 280 + 20 = 300
Step 16: 300 + 20 = 320
Step 17: 320 + 20 = 340
Step 18: 340 + 20 = 360
Step 19: 360 + 20 = 380
Step 20: 380 + 20 = 400
Step 21: 400 + 20 = 420

So, 20*21 = 420