There are seven lights in a circle Ė as shown in
the upper illustration. They are all turned off and the problem is to
turn all of them on.
Each light has a switch. The trouble is that the electrician didnít
use proper switches. When you press a switch, if a light is on it
turns it off, and if a light is off, it turns it on. The electrician
also made a mistake in writing the switches. If you press the switch
for one light, it not only changes that light, but also the lights on
either side. So, if you pressed the switch for light B in the left
diagram of the bottom Example illustration, lights A and B would turn
on and light C would turn off as shown in the right diagram of that
Can you figure out a way to turn all the lights on?