temperature.c:

float farenheit_to_celcius(float temp)
{
    return (temp - 32.0) * (5.0 / 9.0);
}

main()
{
    float temp = -30;
    float tempInCelcius;

    printf("Enter the temperature in Farenheit:\n");

    scanf("%f", &temp);

    if (temp < -459.67)
        printf("It can't possibly be that cold!\n");
    else
    {  
        tempInCelcius = farenheit_to_celcius(temp);
        printf("In Farenheit: %f\n", temp);
        printf("In Celcius: %f\n", tempInCelcius);
    }
}