OK, let's crack this. We're given 991 and our task is to multiply it by 21. Essentially, we'll be adding 991 to itself 21 times.
Step 1: 0 + 991 = 991
Step 2: 991 + 991 = 1982
Step 3: 1982 + 991 = 2973
Step 4: 2973 + 991 = 3964
Step 5: 3964 + 991 = 4955
Step 6: 4955 + 991 = 5946
Step 7: 5946 + 991 = 6937
Step 8: 6937 + 991 = 7928
Step 9: 7928 + 991 = 8919
Step 10: 8919 + 991 = 9910
Step 11: 9910 + 991 = 10901
Step 12: 10901 + 991 = 11892
Step 13: 11892 + 991 = 12883
Step 14: 12883 + 991 = 13874
Step 15: 13874 + 991 = 14865
Step 16: 14865 + 991 = 15856
Step 17: 15856 + 991 = 16847
Step 18: 16847 + 991 = 17838
Step 19: 17838 + 991 = 18829
Step 20: 18829 + 991 = 19820
Step 21: 19820 + 991 = 20811

So, 991*21 = 20811