OK, let's work through this together. We're starting with 357 and we're going to multiply it by 20, which means we add 357 to itself 20 times.
Step 1: 0 + 357 = 357
Step 2: 357 + 357 = 714
Step 3: 714 + 357 = 1071
Step 4: 1071 + 357 = 1428
Step 5: 1428 + 357 = 1785
Step 6: 1785 + 357 = 2142
Step 7: 2142 + 357 = 2499
Step 8: 2499 + 357 = 2856
Step 9: 2856 + 357 = 3213
Step 10: 3213 + 357 = 3570
Step 11: 3570 + 357 = 3927
Step 12: 3927 + 357 = 4284
Step 13: 4284 + 357 = 4641
Step 14: 4641 + 357 = 4998
Step 15: 4998 + 357 = 5355
Step 16: 5355 + 357 = 5712
Step 17: 5712 + 357 = 6069
Step 18: 6069 + 357 = 6426
Step 19: 6426 + 357 = 6783
Step 20: 6783 + 357 = 7140

So, 357*20 = 7140