I prefer the converted fractions for ease. Your solution while right will confuse most Americans. (1.8x-40c)+32=-40f
Dear fellow Americans: Fun tip, lazy (but inaccurate) math is (2x-40c)+30= -50f which when you get to normal ranges works well enough in a margin of error. Or just learn to enjoy the beautiful thing that is Celsius and stop using/converting to Fahrenheit.