Let's roll up our sleeves and solve this. We have 100 and we're going to multiply it by 45, essentially adding 100 to itself 45 times.
Step 1: 0 + 100 = 100
Step 2: 100 + 100 = 200
Step 3: 200 + 100 = 300
Step 4: 300 + 100 = 400
Step 5: 400 + 100 = 500
Step 6: 500 + 100 = 600
Step 7: 600 + 100 = 700
Step 8: 700 + 100 = 800
Step 9: 800 + 100 = 900
Step 10: 900 + 100 = 1000
Step 11: 1000 + 100 = 1100
Step 12: 1100 + 100 = 1200
Step 13: 1200 + 100 = 1300
Step 14: 1300 + 100 = 1400
Step 15: 1400 + 100 = 1500
Step 16: 1500 + 100 = 1600
Step 17: 1600 + 100 = 1700
Step 18: 1700 + 100 = 1800
Step 19: 1800 + 100 = 1900
Step 20: 1900 + 100 = 2000
Step 21: 2000 + 100 = 2100
Step 22: 2100 + 100 = 2200
Step 23: 2200 + 100 = 2300
Step 24: 2300 + 100 = 2400
Step 25: 2400 + 100 = 2500
Step 26: 2500 + 100 = 2600
Step 27: 2600 + 100 = 2700
Step 28: 2700 + 100 = 2800
Step 29: 2800 + 100 = 2900
Step 30: 2900 + 100 = 3000
Step 31: 3000 + 100 = 3100
Step 32: 3100 + 100 = 3200
Step 33: 3200 + 100 = 3300
Step 34: 3300 + 100 = 3400
Step 35: 3400 + 100 = 3500
Step 36: 3500 + 100 = 3600
Step 37: 3600 + 100 = 3700
Step 38: 3700 + 100 = 3800
Step 39: 3800 + 100 = 3900
Step 40: 3900 + 100 = 4000
Step 41: 4000 + 100 = 4100
Step 42: 4100 + 100 = 4200
Step 43: 4200 + 100 = 4300
Step 44: 4300 + 100 = 4400
Step 45: 4400 + 100 = 4500

So, 100*45 = 4500