Let's roll up our sleeves and solve this. We have 815 and we're going to multiply it by 5, essentially adding 815 to itself 5 times.
Step 1: 0 + 815 = 815
Step 2: 815 + 815 = 1630
Step 3: 1630 + 815 = 2445
Step 4: 2445 + 815 = 3260
Step 5: 3260 + 815 = 4075

So, 815*5 = 4075