Ruby - Methods - Keyword Arguments

def convert_temp(initial, input_scale, output_scale: 'celcius')
        if input_scale == "celcius" && output_scale == "fahrenheit"
            # do something
            return (initial * 9/5) + 32
        elsif input_scale == "celcius" && output_scale == "kelvin"
            # do something
            return initial + 273.15
        elsif input_scale == "kelvin" && output_scale == "fahrenheit"
            # do something
            return (initial – 273.15) * 9/5 + 32
        elsif input_scale == "kelvin" && output_scale == "celcius"
            # do something
            return (initial – 273.15)
        elsif input_scale == "fahrenheit" && output_scale == "celcius"
            # do something
            return (initial – 32) * 5/9
        elsif input_scale == "fahrenheit" && output_scale == "kelvin"
            # do something
            return (initial – 32) * 5/9 + 273.15      
        end
end

answer = convert_temp(300.0,"kelvin")
puts answer