OK, let's work through this together. We're starting with 81 and we're going to multiply it by 15, which means we add 81 to itself 15 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
Step 9: 648 + 81 = 729
Step 10: 729 + 81 = 810
Step 11: 810 + 81 = 891
Step 12: 891 + 81 = 972
Step 13: 972 + 81 = 1053
Step 14: 1053 + 81 = 1134
Step 15: 1134 + 81 = 1215

So, 81*15 = 1215