OK, let's crack this. We're given 871 and our task is to multiply it by 31. Essentially, we'll be adding 871 to itself 31 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
Step 21: 17420 + 871 = 18291
Step 22: 18291 + 871 = 19162
Step 23: 19162 + 871 = 20033
Step 24: 20033 + 871 = 20904
Step 25: 20904 + 871 = 21775
Step 26: 21775 + 871 = 22646
Step 27: 22646 + 871 = 23517
Step 28: 23517 + 871 = 24388
Step 29: 24388 + 871 = 25259
Step 30: 25259 + 871 = 26130
Step 31: 26130 + 871 = 27001

So, 871*31 = 27001