#40 There are four lights
set by Paul Taylor
A machine has four lights labelled A to D, one of which is lit at any time. It also has a red button and a blue button. Pressing a button changes which light is on according to the arrows shown. (An arrow from a letter to itself means the light doesn’t change when that button is pressed.)
If light D is lit, how many different ways are there to press exactly three buttons and end up with A lit?
If the last button pressed was blue, what light cannot be lit?…
