temperature_expected.rb

class Temperature
  KelvinToCelsius    = 27315/100r
  KelvinToFahrenheit = 45967/100r
  FahrenheitFactor   = 5/9r

  def self.from_kelvin(kelvin) # could just alias
    new(kelvin: kelvin)
  end

  def self.from_celsius(celsius) # IMO should be just named "celsius"
    new(celsius: celsius)
  end

  def self.from_fahrenheit(fahrenheit)
    new(fahrenheit: fahrenheit)
  end

  attr_reader :in_kelvin

  def initialize(kelvin: nil, celsius: nil, fahrenheit: nil)
    if kelvin && celsius || celsius && fahrenheit || kelvin && fahrenheit
      raise ArgumentError, "Must only pass one of kelvin, celsius or fahrenheit"
    elsif kelvin
      @in_kelvin = kelvin
    elsif celsius
      @in_kelvin = celsius - KelvinToCelsius)
    elsif fahrenheit
      @in_kelvin = (fahrenheit + KelvinToFahrenheit) * FahrenheitFactor)
    else
      raise ArgumentError, "Must only pass at least one of kelvin, celsius or fahrenheit"
    end
  end

  def in_celsius
    @in_kelvin + KelvinToCelsius
  end

  def in_fahrenheit
    (@in_kelvin / FahrenheitFactor) + KelvinToFahrenheit
  end
end

temperature_proper.rb

# proper implementation
class Temperature
  KelvinToCelsius    = 27315/100r
  KelvinToFahrenheit = 45967/100r
  FahrenheitFactor   = 5/9r

  def self.from_kelvin(kelvin) # could just alias
    new(kelvin)
  end

  def self.from_celsius(celsius) # IMO should be just named "celsius"
    new(celsius - KelvinToCelsius)
  end

  def self.from_fahrenheit(fahrenheit)
    new((fahrenheit + KelvinToFahrenheit) * FahrenheitFactor)
  end

  attr_reader :in_kelvin

  def initialize(kelvin)
    @in_kelvin = kelvin
  end

  def in_celsius
    @in_kelvin + KelvinToCelsius
  end

  def in_fahrenheit
    (@in_kelvin / FahrenheitFactor) + KelvinToFahrenheit
  end
end