Sure thing! We've got 639 and we're gonna multiply it by 67. That's the same as adding 639 to itself 67 times.
Step 1: 0 + 639 = 639
Step 2: 639 + 639 = 1278
Step 3: 1278 + 639 = 1917
Step 4: 1917 + 639 = 2556
Step 5: 2556 + 639 = 3195
Step 6: 3195 + 639 = 3834
Step 7: 3834 + 639 = 4473
Step 8: 4473 + 639 = 5112
Step 9: 5112 + 639 = 5751
Step 10: 5751 + 639 = 6390
Step 11: 6390 + 639 = 7029
Step 12: 7029 + 639 = 7668
Step 13: 7668 + 639 = 8307
Step 14: 8307 + 639 = 8946
Step 15: 8946 + 639 = 9585
Step 16: 9585 + 639 = 10224
Step 17: 10224 + 639 = 10863
Step 18: 10863 + 639 = 11502
Step 19: 11502 + 639 = 12141
Step 20: 12141 + 639 = 12780
Step 21: 12780 + 639 = 13419
Step 22: 13419 + 639 = 14058
Step 23: 14058 + 639 = 14697
Step 24: 14697 + 639 = 15336
Step 25: 15336 + 639 = 15975
Step 26: 15975 + 639 = 16614
Step 27: 16614 + 639 = 17253
Step 28: 17253 + 639 = 17892
Step 29: 17892 + 639 = 18531
Step 30: 18531 + 639 = 19170
Step 31: 19170 + 639 = 19809
Step 32: 19809 + 639 = 20448
Step 33: 20448 + 639 = 21087
Step 34: 21087 + 639 = 21726
Step 35: 21726 + 639 = 22365
Step 36: 22365 + 639 = 23004
Step 37: 23004 + 639 = 23643
Step 38: 23643 + 639 = 24282
Step 39: 24282 + 639 = 24921
Step 40: 24921 + 639 = 25560
Step 41: 25560 + 639 = 26199
Step 42: 26199 + 639 = 26838
Step 43: 26838 + 639 = 27477
Step 44: 27477 + 639 = 28116
Step 45: 28116 + 639 = 28755
Step 46: 28755 + 639 = 29394
Step 47: 29394 + 639 = 30033
Step 48: 30033 + 639 = 30672
Step 49: 30672 + 639 = 31311
Step 50: 31311 + 639 = 31950
Step 51: 31950 + 639 = 32589
Step 52: 32589 + 639 = 33228
Step 53: 33228 + 639 = 33867
Step 54: 33867 + 639 = 34506
Step 55: 34506 + 639 = 35145
Step 56: 35145 + 639 = 35784
Step 57: 35784 + 639 = 36423
Step 58: 36423 + 639 = 37062
Step 59: 37062 + 639 = 37701
Step 60: 37701 + 639 = 38340
Step 61: 38340 + 639 = 38979
Step 62: 38979 + 639 = 39618
Step 63: 39618 + 639 = 40257
Step 64: 40257 + 639 = 40896
Step 65: 40896 + 639 = 41535
Step 66: 41535 + 639 = 42174
Step 67: 42174 + 639 = 42813

So, 639*67 = 42813