Without delay, let's solve this. We've got 900 and we will be multiplying it by 4, that is, adding 900 to itself 4 times.
Step 1: 0 + 900 = 900
Step 2: 900 + 900 = 1800
Step 3: 1800 + 900 = 2700
Step 4: 2700 + 900 = 3600

So, 900*4 = 3600