Let's get this math done. We have 99 and we're going to multiply it by 10. This is the same as taking 99 and adding it to itself 10 times.
Step 1: 0 + 99 = 99
Step 2: 99 + 99 = 198
Step 3: 198 + 99 = 297
Step 4: 297 + 99 = 396
Step 5: 396 + 99 = 495
Step 6: 495 + 99 = 594
Step 7: 594 + 99 = 693
Step 8: 693 + 99 = 792
Step 9: 792 + 99 = 891
Step 10: 891 + 99 = 990

So, 99*10 = 990