Each switch toggles two lights: A:1,2; B:2,4; C:1,3; D:3,4. Throw C, B, D, A, but one switch is faulty and has no effect. The left figure becomes the right. Which switch is faulty? Black=on, white=off.
In A B C D E F G H, find the letter two right of the letter immediately left of the letter three right of the letter midway between the letter two left of C and the letter immediately right of F.