OK, let's work through this together. We're starting with 939 and we're going to multiply it by 20, which means we add 939 to itself 20 times.
Step 1: 0 + 939 = 939
Step 2: 939 + 939 = 1878
Step 3: 1878 + 939 = 2817
Step 4: 2817 + 939 = 3756
Step 5: 3756 + 939 = 4695
Step 6: 4695 + 939 = 5634
Step 7: 5634 + 939 = 6573
Step 8: 6573 + 939 = 7512
Step 9: 7512 + 939 = 8451
Step 10: 8451 + 939 = 9390
Step 11: 9390 + 939 = 10329
Step 12: 10329 + 939 = 11268
Step 13: 11268 + 939 = 12207
Step 14: 12207 + 939 = 13146
Step 15: 13146 + 939 = 14085
Step 16: 14085 + 939 = 15024
Step 17: 15024 + 939 = 15963
Step 18: 15963 + 939 = 16902
Step 19: 16902 + 939 = 17841
Step 20: 17841 + 939 = 18780

So, 939*20 = 18780