Let's roll up our sleeves and solve this. We have 310 and we're going to multiply it by 90, essentially adding 310 to itself 90 times.
Step 1: 0 + 310 = 310
Step 2: 310 + 310 = 620
Step 3: 620 + 310 = 930
Step 4: 930 + 310 = 1240
Step 5: 1240 + 310 = 1550
Step 6: 1550 + 310 = 1860
Step 7: 1860 + 310 = 2170
Step 8: 2170 + 310 = 2480
Step 9: 2480 + 310 = 2790
Step 10: 2790 + 310 = 3100
Step 11: 3100 + 310 = 3410
Step 12: 3410 + 310 = 3720
Step 13: 3720 + 310 = 4030
Step 14: 4030 + 310 = 4340
Step 15: 4340 + 310 = 4650
Step 16: 4650 + 310 = 4960
Step 17: 4960 + 310 = 5270
Step 18: 5270 + 310 = 5580
Step 19: 5580 + 310 = 5890
Step 20: 5890 + 310 = 6200
Step 21: 6200 + 310 = 6510
Step 22: 6510 + 310 = 6820
Step 23: 6820 + 310 = 7130
Step 24: 7130 + 310 = 7440
Step 25: 7440 + 310 = 7750
Step 26: 7750 + 310 = 8060
Step 27: 8060 + 310 = 8370
Step 28: 8370 + 310 = 8680
Step 29: 8680 + 310 = 8990
Step 30: 8990 + 310 = 9300
Step 31: 9300 + 310 = 9610
Step 32: 9610 + 310 = 9920
Step 33: 9920 + 310 = 10230
Step 34: 10230 + 310 = 10540
Step 35: 10540 + 310 = 10850
Step 36: 10850 + 310 = 11160
Step 37: 11160 + 310 = 11470
Step 38: 11470 + 310 = 11780
Step 39: 11780 + 310 = 12090
Step 40: 12090 + 310 = 12400
Step 41: 12400 + 310 = 12710
Step 42: 12710 + 310 = 13020
Step 43: 13020 + 310 = 13330
Step 44: 13330 + 310 = 13640
Step 45: 13640 + 310 = 13950
Step 46: 13950 + 310 = 14260
Step 47: 14260 + 310 = 14570
Step 48: 14570 + 310 = 14880
Step 49: 14880 + 310 = 15190
Step 50: 15190 + 310 = 15500
Step 51: 15500 + 310 = 15810
Step 52: 15810 + 310 = 16120
Step 53: 16120 + 310 = 16430
Step 54: 16430 + 310 = 16740
Step 55: 16740 + 310 = 17050
Step 56: 17050 + 310 = 17360
Step 57: 17360 + 310 = 17670
Step 58: 17670 + 310 = 17980
Step 59: 17980 + 310 = 18290
Step 60: 18290 + 310 = 18600
Step 61: 18600 + 310 = 18910
Step 62: 18910 + 310 = 19220
Step 63: 19220 + 310 = 19530
Step 64: 19530 + 310 = 19840
Step 65: 19840 + 310 = 20150
Step 66: 20150 + 310 = 20460
Step 67: 20460 + 310 = 20770
Step 68: 20770 + 310 = 21080
Step 69: 21080 + 310 = 21390
Step 70: 21390 + 310 = 21700
Step 71: 21700 + 310 = 22010
Step 72: 22010 + 310 = 22320
Step 73: 22320 + 310 = 22630
Step 74: 22630 + 310 = 22940
Step 75: 22940 + 310 = 23250
Step 76: 23250 + 310 = 23560
Step 77: 23560 + 310 = 23870
Step 78: 23870 + 310 = 24180
Step 79: 24180 + 310 = 24490
Step 80: 24490 + 310 = 24800
Step 81: 24800 + 310 = 25110
Step 82: 25110 + 310 = 25420
Step 83: 25420 + 310 = 25730
Step 84: 25730 + 310 = 26040
Step 85: 26040 + 310 = 26350
Step 86: 26350 + 310 = 26660
Step 87: 26660 + 310 = 26970
Step 88: 26970 + 310 = 27280
Step 89: 27280 + 310 = 27590
Step 90: 27590 + 310 = 27900

So, 310*90 = 27900