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

+-----+       +-----+
|  1  |       |  1  |
| OFF |       | OFF |
+-----+       +-----+
|  2  |       |  2  |
| OFF |       | ON  |
+-----+       +-----+
|  3  |       |  3  |
| OFF |       | OFF |
+-----+       +-----+
|  4  |       |  4  |
| OFF |       | ON  |
+-----+       +-----+

FIGURE 1      FIGURE 2

Switches D, B, C and A are thrown in turn with result that Figure 1 turns into Figure 2. Each cell represents a given light and its state.

One of the switches does not work at all. Which one?

(In reply to Solution (different from previous) by Jer)

You are right,

I used the same reasoning, but wrote the result much later..

remembering A INSTEAD OF B.

Posted by Ady TZIDON on 2014-10-20 12:03:10