Turn two machine switches off, go outside the room and, by listening, test all six combinations of two back-up switches to find which two match the two "machine-on" switches inside the room. Only one of the six combinations of two back-ups gives complete silence when switched off and these "machine-on" back-ups match the two "on" in the room. Now, before returning to the room, leave one "on" off and one "off" on leaving the other "on" on and "off" off, that is, only two switches are activated. Back inside the room, we can now match the switches by seeing which "on" machine is now off and which "off" is now on (when we activate its switch).