OK, let's crack this. We're given 81 and our task is to multiply it by 5. Essentially, we'll be adding 81 to itself 5 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

So, 81*5 = 405