Let's roll up our sleeves and solve this. We have 420 and we're going to multiply it by 5, essentially adding 420 to itself 5 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

So, 420*5 = 2100