Sure thing, let's get straight to it. We start with 100 and we're going to multiply it by 21, which means adding 100 to itself 21 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

So, 100*21 = 2100