Without delay, let's solve this. We've got 40 and we will be multiplying it by 90, that is, adding 40 to itself 90 times.
Step 1: 0 + 40 = 40
Step 2: 40 + 40 = 80
Step 3: 80 + 40 = 120
Step 4: 120 + 40 = 160
Step 5: 160 + 40 = 200
Step 6: 200 + 40 = 240
Step 7: 240 + 40 = 280
Step 8: 280 + 40 = 320
Step 9: 320 + 40 = 360
Step 10: 360 + 40 = 400
Step 11: 400 + 40 = 440
Step 12: 440 + 40 = 480
Step 13: 480 + 40 = 520
Step 14: 520 + 40 = 560
Step 15: 560 + 40 = 600
Step 16: 600 + 40 = 640
Step 17: 640 + 40 = 680
Step 18: 680 + 40 = 720
Step 19: 720 + 40 = 760
Step 20: 760 + 40 = 800
Step 21: 800 + 40 = 840
Step 22: 840 + 40 = 880
Step 23: 880 + 40 = 920
Step 24: 920 + 40 = 960
Step 25: 960 + 40 = 1000
Step 26: 1000 + 40 = 1040
Step 27: 1040 + 40 = 1080
Step 28: 1080 + 40 = 1120
Step 29: 1120 + 40 = 1160
Step 30: 1160 + 40 = 1200
Step 31: 1200 + 40 = 1240
Step 32: 1240 + 40 = 1280
Step 33: 1280 + 40 = 1320
Step 34: 1320 + 40 = 1360
Step 35: 1360 + 40 = 1400
Step 36: 1400 + 40 = 1440
Step 37: 1440 + 40 = 1480
Step 38: 1480 + 40 = 1520
Step 39: 1520 + 40 = 1560
Step 40: 1560 + 40 = 1600
Step 41: 1600 + 40 = 1640
Step 42: 1640 + 40 = 1680
Step 43: 1680 + 40 = 1720
Step 44: 1720 + 40 = 1760
Step 45: 1760 + 40 = 1800
Step 46: 1800 + 40 = 1840
Step 47: 1840 + 40 = 1880
Step 48: 1880 + 40 = 1920
Step 49: 1920 + 40 = 1960
Step 50: 1960 + 40 = 2000
Step 51: 2000 + 40 = 2040
Step 52: 2040 + 40 = 2080
Step 53: 2080 + 40 = 2120
Step 54: 2120 + 40 = 2160
Step 55: 2160 + 40 = 2200
Step 56: 2200 + 40 = 2240
Step 57: 2240 + 40 = 2280
Step 58: 2280 + 40 = 2320
Step 59: 2320 + 40 = 2360
Step 60: 2360 + 40 = 2400
Step 61: 2400 + 40 = 2440
Step 62: 2440 + 40 = 2480
Step 63: 2480 + 40 = 2520
Step 64: 2520 + 40 = 2560
Step 65: 2560 + 40 = 2600
Step 66: 2600 + 40 = 2640
Step 67: 2640 + 40 = 2680
Step 68: 2680 + 40 = 2720
Step 69: 2720 + 40 = 2760
Step 70: 2760 + 40 = 2800
Step 71: 2800 + 40 = 2840
Step 72: 2840 + 40 = 2880
Step 73: 2880 + 40 = 2920
Step 74: 2920 + 40 = 2960
Step 75: 2960 + 40 = 3000
Step 76: 3000 + 40 = 3040
Step 77: 3040 + 40 = 3080
Step 78: 3080 + 40 = 3120
Step 79: 3120 + 40 = 3160
Step 80: 3160 + 40 = 3200
Step 81: 3200 + 40 = 3240
Step 82: 3240 + 40 = 3280
Step 83: 3280 + 40 = 3320
Step 84: 3320 + 40 = 3360
Step 85: 3360 + 40 = 3400
Step 86: 3400 + 40 = 3440
Step 87: 3440 + 40 = 3480
Step 88: 3480 + 40 = 3520
Step 89: 3520 + 40 = 3560
Step 90: 3560 + 40 = 3600

So, 40*90 = 3600