Assuming an atmospheric pressure of 1 atmosphere, how cold does it have to be to keep your lighter from working? (i.e. At what temperature is the pressure of butane equal to 1 atmosphere?)
In case you've forgotten the value of DHvap, at 27 °C the vapor pressure of isobutane is 2,775 torr and at -3 °C it is 1050 torr, so you can figure it out again.