Alright, let's solve this math problem step by step. First, we have the number 400. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 15. So, we're going to take the number 400 and add it to itself 15 times.
Step 1: 0 + 400 = 400
Step 2: 400 + 400 = 800
Step 3: 800 + 400 = 1200
Step 4: 1200 + 400 = 1600
Step 5: 1600 + 400 = 2000
Step 6: 2000 + 400 = 2400
Step 7: 2400 + 400 = 2800
Step 8: 2800 + 400 = 3200
Step 9: 3200 + 400 = 3600
Step 10: 3600 + 400 = 4000
Step 11: 4000 + 400 = 4400
Step 12: 4400 + 400 = 4800
Step 13: 4800 + 400 = 5200
Step 14: 5200 + 400 = 5600
Step 15: 5600 + 400 = 6000

So, 400*15 = 6000