Sure thing! We've got 640 and we're gonna multiply it by 31. That's the same as adding 640 to itself 31 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
Step 21: 12800 + 640 = 13440
Step 22: 13440 + 640 = 14080
Step 23: 14080 + 640 = 14720
Step 24: 14720 + 640 = 15360
Step 25: 15360 + 640 = 16000
Step 26: 16000 + 640 = 16640
Step 27: 16640 + 640 = 17280
Step 28: 17280 + 640 = 17920
Step 29: 17920 + 640 = 18560
Step 30: 18560 + 640 = 19200
Step 31: 19200 + 640 = 19840

So, 640*31 = 19840