Sure thing, let's get straight to it. We start with 640 and we're going to multiply it by 20, which means adding 640 to itself 20 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
Step 7: 3840 + 640 = 4480
Step 8: 4480 + 640 = 5120
Step 9: 5120 + 640 = 5760
Step 10: 5760 + 640 = 6400
Step 11: 6400 + 640 = 7040
Step 12: 7040 + 640 = 7680
Step 13: 7680 + 640 = 8320
Step 14: 8320 + 640 = 8960
Step 15: 8960 + 640 = 9600
Step 16: 9600 + 640 = 10240
Step 17: 10240 + 640 = 10880
Step 18: 10880 + 640 = 11520
Step 19: 11520 + 640 = 12160
Step 20: 12160 + 640 = 12800

So, 640*20 = 12800