You have already said it is 100 F for planet A.
Perhaps you mistyped the question.
planet A has an average surface temperature of -100F. Planet B has an average surface temperature that is 3/2 times of that Planet A. What is the average surface temperature of Planet A?
2 answers
It makes no sense physically to talk about one temperature being a certain fraction of another unless absolute (Kelvin or Rankine) temperatures are used. Such temperatures newer go below zero.
The Fahrenheit and Celsius scales do not measure absolute temperature, as you know.
The Fahrenheit and Celsius scales do not measure absolute temperature, as you know.