Let's roll up our sleeves and solve this. We have 871 and we're going to multiply it by 20, essentially adding 871 to itself 20 times.
Step 1: 0 + 871 = 871
Step 2: 871 + 871 = 1742
Step 3: 1742 + 871 = 2613
Step 4: 2613 + 871 = 3484
Step 5: 3484 + 871 = 4355
Step 6: 4355 + 871 = 5226
Step 7: 5226 + 871 = 6097
Step 8: 6097 + 871 = 6968
Step 9: 6968 + 871 = 7839
Step 10: 7839 + 871 = 8710
Step 11: 8710 + 871 = 9581
Step 12: 9581 + 871 = 10452
Step 13: 10452 + 871 = 11323
Step 14: 11323 + 871 = 12194
Step 15: 12194 + 871 = 13065
Step 16: 13065 + 871 = 13936
Step 17: 13936 + 871 = 14807
Step 18: 14807 + 871 = 15678
Step 19: 15678 + 871 = 16549
Step 20: 16549 + 871 = 17420

So, 871*20 = 17420