If you wish to save fuel and you're going to leave your cool house for a half hour or so on a very hot day, should you turn your air-conditioning thermostat up a bit, turn it off altogether, or let it remain at the room temperature you desire?

turn it off altogether

Explanation:

          In order to save fuel when we want our house to remain cool for half and hour  or so on a very hot day, we should totally turn the air conditioner off altogether.

          Keeping the air conditioner switched on for a longer period of time will used fuel or the refrigerant. The air conditioner turned on can make the room cool enough for some time longer even after the air conditioner is turned off. This helps in saving the fuel.

         Otherwise if we keep the air conditioner on for more, the room will become more cold and heat can leak at a faster rate into the more cold room than not-so cold room. Thus the fuel consumption will be more, if the more heat leaks into the room.

Thus the answer is 'turn it off altogether".