Let's roll up our sleeves and solve this. We have 990 and we're going to multiply it by 100, essentially adding 990 to itself 100 times.
Step 1: 0 + 990 = 990
Step 2: 990 + 990 = 1980
Step 3: 1980 + 990 = 2970
Step 4: 2970 + 990 = 3960
Step 5: 3960 + 990 = 4950
Step 6: 4950 + 990 = 5940
Step 7: 5940 + 990 = 6930
Step 8: 6930 + 990 = 7920
Step 9: 7920 + 990 = 8910
Step 10: 8910 + 990 = 9900
Step 11: 9900 + 990 = 10890
Step 12: 10890 + 990 = 11880
Step 13: 11880 + 990 = 12870
Step 14: 12870 + 990 = 13860
Step 15: 13860 + 990 = 14850
Step 16: 14850 + 990 = 15840
Step 17: 15840 + 990 = 16830
Step 18: 16830 + 990 = 17820
Step 19: 17820 + 990 = 18810
Step 20: 18810 + 990 = 19800
Step 21: 19800 + 990 = 20790
Step 22: 20790 + 990 = 21780
Step 23: 21780 + 990 = 22770
Step 24: 22770 + 990 = 23760
Step 25: 23760 + 990 = 24750
Step 26: 24750 + 990 = 25740
Step 27: 25740 + 990 = 26730
Step 28: 26730 + 990 = 27720
Step 29: 27720 + 990 = 28710
Step 30: 28710 + 990 = 29700
Step 31: 29700 + 990 = 30690
Step 32: 30690 + 990 = 31680
Step 33: 31680 + 990 = 32670
Step 34: 32670 + 990 = 33660
Step 35: 33660 + 990 = 34650
Step 36: 34650 + 990 = 35640
Step 37: 35640 + 990 = 36630
Step 38: 36630 + 990 = 37620
Step 39: 37620 + 990 = 38610
Step 40: 38610 + 990 = 39600
Step 41: 39600 + 990 = 40590
Step 42: 40590 + 990 = 41580
Step 43: 41580 + 990 = 42570
Step 44: 42570 + 990 = 43560
Step 45: 43560 + 990 = 44550
Step 46: 44550 + 990 = 45540
Step 47: 45540 + 990 = 46530
Step 48: 46530 + 990 = 47520
Step 49: 47520 + 990 = 48510
Step 50: 48510 + 990 = 49500
Step 51: 49500 + 990 = 50490
Step 52: 50490 + 990 = 51480
Step 53: 51480 + 990 = 52470
Step 54: 52470 + 990 = 53460
Step 55: 53460 + 990 = 54450
Step 56: 54450 + 990 = 55440
Step 57: 55440 + 990 = 56430
Step 58: 56430 + 990 = 57420
Step 59: 57420 + 990 = 58410
Step 60: 58410 + 990 = 59400
Step 61: 59400 + 990 = 60390
Step 62: 60390 + 990 = 61380
Step 63: 61380 + 990 = 62370
Step 64: 62370 + 990 = 63360
Step 65: 63360 + 990 = 64350
Step 66: 64350 + 990 = 65340
Step 67: 65340 + 990 = 66330
Step 68: 66330 + 990 = 67320
Step 69: 67320 + 990 = 68310
Step 70: 68310 + 990 = 69300
Step 71: 69300 + 990 = 70290
Step 72: 70290 + 990 = 71280
Step 73: 71280 + 990 = 72270
Step 74: 72270 + 990 = 73260
Step 75: 73260 + 990 = 74250
Step 76: 74250 + 990 = 75240
Step 77: 75240 + 990 = 76230
Step 78: 76230 + 990 = 77220
Step 79: 77220 + 990 = 78210
Step 80: 78210 + 990 = 79200
Step 81: 79200 + 990 = 80190
Step 82: 80190 + 990 = 81180
Step 83: 81180 + 990 = 82170
Step 84: 82170 + 990 = 83160
Step 85: 83160 + 990 = 84150
Step 86: 84150 + 990 = 85140
Step 87: 85140 + 990 = 86130
Step 88: 86130 + 990 = 87120
Step 89: 87120 + 990 = 88110
Step 90: 88110 + 990 = 89100
Step 91: 89100 + 990 = 90090
Step 92: 90090 + 990 = 91080
Step 93: 91080 + 990 = 92070
Step 94: 92070 + 990 = 93060
Step 95: 93060 + 990 = 94050
Step 96: 94050 + 990 = 95040
Step 97: 95040 + 990 = 96030
Step 98: 96030 + 990 = 97020
Step 99: 97020 + 990 = 98010
Step 100: 98010 + 990 = 99000

So, 990*100 = 99000