FL290 ISA       = +15º - (2º x 29)         Pressure altitude         = 29,000 ft

                                    = -43ºC            Diff = 120 ft x -12ºC    =  -1,440 ft

                        OAT    = -55ºC             Density Altitude          = 27,560 ft

            ISA deviation  = -12ºC