Let's roll up our sleeves and solve this. We have 31 and we're going to multiply it by 28, essentially adding 31 to itself 28 times.
Step 1: 0 + 31 = 31
Step 2: 31 + 31 = 62
Step 3: 62 + 31 = 93
Step 4: 93 + 31 = 124
Step 5: 124 + 31 = 155
Step 6: 155 + 31 = 186
Step 7: 186 + 31 = 217
Step 8: 217 + 31 = 248
Step 9: 248 + 31 = 279
Step 10: 279 + 31 = 310
Step 11: 310 + 31 = 341
Step 12: 341 + 31 = 372
Step 13: 372 + 31 = 403
Step 14: 403 + 31 = 434
Step 15: 434 + 31 = 465
Step 16: 465 + 31 = 496
Step 17: 496 + 31 = 527
Step 18: 527 + 31 = 558
Step 19: 558 + 31 = 589
Step 20: 589 + 31 = 620
Step 21: 620 + 31 = 651
Step 22: 651 + 31 = 682
Step 23: 682 + 31 = 713
Step 24: 713 + 31 = 744
Step 25: 744 + 31 = 775
Step 26: 775 + 31 = 806
Step 27: 806 + 31 = 837
Step 28: 837 + 31 = 868

So, 31*28 = 868