Let's get this math done. We have 420 and we're going to multiply it by 15. This is the same as taking 420 and adding it to itself 15 times.
Step 1: 0 + 420 = 420
Step 2: 420 + 420 = 840
Step 3: 840 + 420 = 1260
Step 4: 1260 + 420 = 1680
Step 5: 1680 + 420 = 2100
Step 6: 2100 + 420 = 2520
Step 7: 2520 + 420 = 2940
Step 8: 2940 + 420 = 3360
Step 9: 3360 + 420 = 3780
Step 10: 3780 + 420 = 4200
Step 11: 4200 + 420 = 4620
Step 12: 4620 + 420 = 5040
Step 13: 5040 + 420 = 5460
Step 14: 5460 + 420 = 5880
Step 15: 5880 + 420 = 6300

So, 420*15 = 6300