Let's roll up our sleeves and solve this. We have 771 and we're going to multiply it by 15, essentially adding 771 to itself 15 times.
Step 1: 0 + 771 = 771
Step 2: 771 + 771 = 1542
Step 3: 1542 + 771 = 2313
Step 4: 2313 + 771 = 3084
Step 5: 3084 + 771 = 3855
Step 6: 3855 + 771 = 4626
Step 7: 4626 + 771 = 5397
Step 8: 5397 + 771 = 6168
Step 9: 6168 + 771 = 6939
Step 10: 6939 + 771 = 7710
Step 11: 7710 + 771 = 8481
Step 12: 8481 + 771 = 9252
Step 13: 9252 + 771 = 10023
Step 14: 10023 + 771 = 10794
Step 15: 10794 + 771 = 11565

So, 771*15 = 11565