When i choose 4 mars why does it start at 0.4992 and not 49.92 what do i do?

Solution 1:

You should use either fractions (say * 0.39) or per cents (* 39) / 100), but not a combination:

Your code amended:

        static double convert(int choice, double on_earth)
        {
            double on_planet = 0;
            if (choice == 1)
                on_planet = (on_earth * 0.38);
            else if (choice == 2)
                on_planet = (on_earth * 0.78);
            else if (choice == 3)
                on_planet = (on_earth * 1.0);
            else if (choice == 4)
                on_planet = (on_earth * 0.39);
            else if (choice == 5)
                on_planet = (on_earth * 2.65);
            else if (choice == 6)
                on_planet = (on_earth * 1.17);
            else if (choice == 7)
                on_planet = (on_earth * 1.05);
            else if (choice == 8)
                on_planet = (on_earth * 1.23);
            return on_planet;
        }

A better choice is to extract model:

// With model extracted it's much easier to see the wrong figures
static double[] s_OnPlanets = new double[] {
  0.38, 0.78, 1.00, 0.39, 2.65, 1.17, 1.05, 1.23
};

// Now we have no need in that many ifs:
static double convert(int choice, double on_earth) =>
  s_OnPlanets[choice - 1] * on_earth;