Without delay, let's solve this. We've got 755 and we will be multiplying it by 21, that is, adding 755 to itself 21 times.
Step 1: 0 + 755 = 755
Step 2: 755 + 755 = 1510
Step 3: 1510 + 755 = 2265
Step 4: 2265 + 755 = 3020
Step 5: 3020 + 755 = 3775
Step 6: 3775 + 755 = 4530
Step 7: 4530 + 755 = 5285
Step 8: 5285 + 755 = 6040
Step 9: 6040 + 755 = 6795
Step 10: 6795 + 755 = 7550
Step 11: 7550 + 755 = 8305
Step 12: 8305 + 755 = 9060
Step 13: 9060 + 755 = 9815
Step 14: 9815 + 755 = 10570
Step 15: 10570 + 755 = 11325
Step 16: 11325 + 755 = 12080
Step 17: 12080 + 755 = 12835
Step 18: 12835 + 755 = 13590
Step 19: 13590 + 755 = 14345
Step 20: 14345 + 755 = 15100
Step 21: 15100 + 755 = 15855

So, 755*21 = 15855