A system of switches A, B, C and D control a set of four lights. When each switch is working, it has the following effects on the lights:
Switch A turns lights 1 and 2 on/off or off/on
Switch B turns lights 2 and 4 on/off or off/on
Switch C turns lights 1 and 3 on/off or off/on
Switch D turns lights 3 and 4 on/off or off/on
In the following diagram, switches C B D A are thrown in turn, with the result that Figure 1 is transformed into Figure 2. Therefore, one of the switches is not working and has had no effect on the numbered lights.