Let's roll up our sleeves and solve this. We have 800 and we're going to multiply it by 13, essentially adding 800 to itself 13 times.
Step 1: 0 + 800 = 800
Step 2: 800 + 800 = 1600
Step 3: 1600 + 800 = 2400
Step 4: 2400 + 800 = 3200
Step 5: 3200 + 800 = 4000
Step 6: 4000 + 800 = 4800
Step 7: 4800 + 800 = 5600
Step 8: 5600 + 800 = 6400
Step 9: 6400 + 800 = 7200
Step 10: 7200 + 800 = 8000
Step 11: 8000 + 800 = 8800
Step 12: 8800 + 800 = 9600
Step 13: 9600 + 800 = 10400

So, 800*13 = 10400