Top two switches are least significant bit and bottom two switches are most significant bit. The LEDs are least significant bit on right and most on left. So top switches are worth one middle two switches are worth 2 and bottom two switches are worth 4. So if you have 101 + 011 or (5+3) you would get 1000 or 8. Since the first led is most significant bit, it equals 8 and it will be the only one lit.
