OK, let's crack this. We're given 278 and our task is to multiply it by 21. Essentially, we'll be adding 278 to itself 21 times.
Step 1: 0 + 278 = 278
Step 2: 278 + 278 = 556
Step 3: 556 + 278 = 834
Step 4: 834 + 278 = 1112
Step 5: 1112 + 278 = 1390
Step 6: 1390 + 278 = 1668
Step 7: 1668 + 278 = 1946
Step 8: 1946 + 278 = 2224
Step 9: 2224 + 278 = 2502
Step 10: 2502 + 278 = 2780
Step 11: 2780 + 278 = 3058
Step 12: 3058 + 278 = 3336
Step 13: 3336 + 278 = 3614
Step 14: 3614 + 278 = 3892
Step 15: 3892 + 278 = 4170
Step 16: 4170 + 278 = 4448
Step 17: 4448 + 278 = 4726
Step 18: 4726 + 278 = 5004
Step 19: 5004 + 278 = 5282
Step 20: 5282 + 278 = 5560
Step 21: 5560 + 278 = 5838

So, 278*21 = 5838