OK, let's crack this. We're given 640 and our task is to multiply it by 6. Essentially, we'll be adding 640 to itself 6 times.
Step 1: 0 + 640 = 640
Step 2: 640 + 640 = 1280
Step 3: 1280 + 640 = 1920
Step 4: 1920 + 640 = 2560
Step 5: 2560 + 640 = 3200
Step 6: 3200 + 640 = 3840

So, 640*6 = 3840