OK, let's work through this together. We're starting with 904 and we're going to multiply it by 15, which means we add 904 to itself 15 times.
Step 1: 0 + 904 = 904
Step 2: 904 + 904 = 1808
Step 3: 1808 + 904 = 2712
Step 4: 2712 + 904 = 3616
Step 5: 3616 + 904 = 4520
Step 6: 4520 + 904 = 5424
Step 7: 5424 + 904 = 6328
Step 8: 6328 + 904 = 7232
Step 9: 7232 + 904 = 8136
Step 10: 8136 + 904 = 9040
Step 11: 9040 + 904 = 9944
Step 12: 9944 + 904 = 10848
Step 13: 10848 + 904 = 11752
Step 14: 11752 + 904 = 12656
Step 15: 12656 + 904 = 13560

So, 904*15 = 13560