OK, let's crack this. We're given 129 and our task is to multiply it by 31. Essentially, we'll be adding 129 to itself 31 times.
Step 1: 0 + 129 = 129
Step 2: 129 + 129 = 258
Step 3: 258 + 129 = 387
Step 4: 387 + 129 = 516
Step 5: 516 + 129 = 645
Step 6: 645 + 129 = 774
Step 7: 774 + 129 = 903
Step 8: 903 + 129 = 1032
Step 9: 1032 + 129 = 1161
Step 10: 1161 + 129 = 1290
Step 11: 1290 + 129 = 1419
Step 12: 1419 + 129 = 1548
Step 13: 1548 + 129 = 1677
Step 14: 1677 + 129 = 1806
Step 15: 1806 + 129 = 1935
Step 16: 1935 + 129 = 2064
Step 17: 2064 + 129 = 2193
Step 18: 2193 + 129 = 2322
Step 19: 2322 + 129 = 2451
Step 20: 2451 + 129 = 2580
Step 21: 2580 + 129 = 2709
Step 22: 2709 + 129 = 2838
Step 23: 2838 + 129 = 2967
Step 24: 2967 + 129 = 3096
Step 25: 3096 + 129 = 3225
Step 26: 3225 + 129 = 3354
Step 27: 3354 + 129 = 3483
Step 28: 3483 + 129 = 3612
Step 29: 3612 + 129 = 3741
Step 30: 3741 + 129 = 3870
Step 31: 3870 + 129 = 3999

So, 129*31 = 3999