OK, let's work through this together. We're starting with 924 and we're going to multiply it by 15, which means we add 924 to itself 15 times.
Step 1: 0 + 924 = 924
Step 2: 924 + 924 = 1848
Step 3: 1848 + 924 = 2772
Step 4: 2772 + 924 = 3696
Step 5: 3696 + 924 = 4620
Step 6: 4620 + 924 = 5544
Step 7: 5544 + 924 = 6468
Step 8: 6468 + 924 = 7392
Step 9: 7392 + 924 = 8316
Step 10: 8316 + 924 = 9240
Step 11: 9240 + 924 = 10164
Step 12: 10164 + 924 = 11088
Step 13: 11088 + 924 = 12012
Step 14: 12012 + 924 = 12936
Step 15: 12936 + 924 = 13860

So, 924*15 = 13860