Let's roll up our sleeves and solve this. We have 878 and we're going to multiply it by 42, essentially adding 878 to itself 42 times.
Step 1: 0 + 878 = 878
Step 2: 878 + 878 = 1756
Step 3: 1756 + 878 = 2634
Step 4: 2634 + 878 = 3512
Step 5: 3512 + 878 = 4390
Step 6: 4390 + 878 = 5268
Step 7: 5268 + 878 = 6146
Step 8: 6146 + 878 = 7024
Step 9: 7024 + 878 = 7902
Step 10: 7902 + 878 = 8780
Step 11: 8780 + 878 = 9658
Step 12: 9658 + 878 = 10536
Step 13: 10536 + 878 = 11414
Step 14: 11414 + 878 = 12292
Step 15: 12292 + 878 = 13170
Step 16: 13170 + 878 = 14048
Step 17: 14048 + 878 = 14926
Step 18: 14926 + 878 = 15804
Step 19: 15804 + 878 = 16682
Step 20: 16682 + 878 = 17560
Step 21: 17560 + 878 = 18438
Step 22: 18438 + 878 = 19316
Step 23: 19316 + 878 = 20194
Step 24: 20194 + 878 = 21072
Step 25: 21072 + 878 = 21950
Step 26: 21950 + 878 = 22828
Step 27: 22828 + 878 = 23706
Step 28: 23706 + 878 = 24584
Step 29: 24584 + 878 = 25462
Step 30: 25462 + 878 = 26340
Step 31: 26340 + 878 = 27218
Step 32: 27218 + 878 = 28096
Step 33: 28096 + 878 = 28974
Step 34: 28974 + 878 = 29852
Step 35: 29852 + 878 = 30730
Step 36: 30730 + 878 = 31608
Step 37: 31608 + 878 = 32486
Step 38: 32486 + 878 = 33364
Step 39: 33364 + 878 = 34242
Step 40: 34242 + 878 = 35120
Step 41: 35120 + 878 = 35998
Step 42: 35998 + 878 = 36876

So, 878*42 = 36876