OK, let's crack this. We're given 638 and our task is to multiply it by 20. Essentially, we'll be adding 638 to itself 20 times.
Step 1: 0 + 638 = 638
Step 2: 638 + 638 = 1276
Step 3: 1276 + 638 = 1914
Step 4: 1914 + 638 = 2552
Step 5: 2552 + 638 = 3190
Step 6: 3190 + 638 = 3828
Step 7: 3828 + 638 = 4466
Step 8: 4466 + 638 = 5104
Step 9: 5104 + 638 = 5742
Step 10: 5742 + 638 = 6380
Step 11: 6380 + 638 = 7018
Step 12: 7018 + 638 = 7656
Step 13: 7656 + 638 = 8294
Step 14: 8294 + 638 = 8932
Step 15: 8932 + 638 = 9570
Step 16: 9570 + 638 = 10208
Step 17: 10208 + 638 = 10846
Step 18: 10846 + 638 = 11484
Step 19: 11484 + 638 = 12122
Step 20: 12122 + 638 = 12760

So, 638*20 = 12760