The Dishonest Gas Shop

A gas seller has learned that most things expand when they are heated, and has decided that as he sells gas by the gallon, he would make a much bigger profit if he heated his wares before selling them.

In this particular case, he has 27 gallons of gas at 65°F, and wants to expand it to 30 gallons. How hot must he make it?

According to the Ideal Gas Law (a chemistry thing), there is a very simple formula:
but that formula only works when temperature is measured in Absolute or Kelvin degrees, not Fahrenheit. So he needs to convert from Fahrenheit to Kelvin... 
      #include "library.h"
      
      int C_to_F(int C)
      {
        return C*9/5+32;
      }
      
      int F_to_C(int F)
      {
        return (F-32)*5/9;
      }
      
      int C_to_K(int C)
      {
        return C+273.15;
      }
      
      int K_to_C(int K)
      {
        return K-273.15;
      }
      
      int F_to_K(int F)
      {
        return C_to_K(F_to_C(F));
      }
      
      int K_to_F(int K)
      {
        return C_to_F(K_to_C(K));
      }
      
      void main(void)
      { ...
        whatever
        ... }
Error! (next)