Let's roll up our sleeves and solve this. We have 81 and we're going to multiply it by 8, essentially adding 81 to itself 8 times.
Step 1: 0 + 81 = 81
Step 2: 81 + 81 = 162
Step 3: 162 + 81 = 243
Step 4: 243 + 81 = 324
Step 5: 324 + 81 = 405
Step 6: 405 + 81 = 486
Step 7: 486 + 81 = 567
Step 8: 567 + 81 = 648

So, 81*8 = 648