math
posted by carmen .
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?

You have already said it is 100 F for planet A.
Perhaps you mistyped the question. 
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.