Sure thing! We've got 100 and we're gonna multiply it by 9. That's the same as adding 100 to itself 9 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

So, 100*9 = 900