Sure thing! We've got 640 and we're gonna multiply it by 4. That's the same as adding 640 to itself 4 times.
Step 1: 0 + 640 = 640
Step 2: 640 + 640 = 1280
Step 3: 1280 + 640 = 1920
Step 4: 1920 + 640 = 2560

So, 640*4 = 2560