Mathematical quiz game I have been studying C#, and I have made a simple math quiz program.

Is my program reasonable or is there anything I need to improve?

class Program
{

    static void Main(string[] args)
    {
        Console.WriteLine("============== Math Game ===================");

        double num1, num2;
        double userInput;
        int randomIndex;
        double sum = 0;
        string operation;

        // [2]
        //int lives = 0;

        // Console.Write("How many lives do you want?\t");
        // lives = Convert.ToInt32(Console.ReadLine());
        // Console.WriteLine("You have  {0} Lives", lives);

        bool isPlaying = true;
        do
        {
            // 1 generate randon#
            Random rnd = new Random();
            randomIndex = rnd.Next(1, 4);
            num1 = rnd.Next(1, 11);
            num2 = rnd.Next(1, 11);

            // 2 cal sum &amp;&amp; operation &amp;&amp; Display
            sum = Cal(num1, num2, randomIndex);
            operation = Operation(randomIndex);

           // DisplayProblem(num1, num2, operation);

            // 3 request input
            userInput = RequestInput(num1,num2,operation);

            // 4 check answer
            CheckAnswer(userInput, sum);


        } while (isPlaying);

    }
    private static double Cal(double num1, double num2, int randomIndex)
    {
        double sum = 0;
        switch (randomIndex)
        {
            case 1:
                sum = num1 + num2;
                break;
            case 2:
                sum = num1 - num2;
                break;
            case 3:
                sum = num1 * num2;
                break;
            case 4:
                sum = num1 / num2;
                break;
            default:
                Console.WriteLine("Error");
                break;
        }
        return sum;
    }
    private static string Operation(int randomIndex)
    {
        string operation = "";

        switch (randomIndex)
        {
            case 1:
                operation = "+";
                break;
            case 2:
                operation = "-";
                break;
            case 3:
                operation = "*";
                break;
            case 4:
                operation = "/";
                break;
            default:
                Console.WriteLine("error");
                break;
        }
        return operation;
    }
    private static void DisplayProblem(double num1, double num2, string operation)
    {
        Console.Write($"{num1} {operation} {num2}  = ");
    }
    private static double RequestInput(double num1, double num2, string operation)
    {
        DisplayProblem(num1, num2, operation);

        double userInput;
        while (!Double.TryParse(Console.ReadLine(),out userInput))
        {
            Console.WriteLine("Invalid input. Please type a whole num");
            DisplayProblem(num1, num2, operation);
        }
        return userInput;
    }
    private static void CheckAnswer(double userInput, double sum)
    {
        if (userInput == sum)
        {
            Console.WriteLine("Correct Answer.");
        }
        else if(userInput != sum)
        {
            Console.WriteLine("Wrong Answer.");
        }
    }
}
In the Operation method

private static string Operation(int randomIndex)
{
    string operation = "";
    switch (randomIndex)
    {
        case 1:
            operation = "+";
            break;
        case 2:
            operation = "-";
            break;
        case 3:
            operation = "*";
            break;
        case 4:
            operation = "/";
            break;
        default:
            Console.WriteLine("error");
            break;
    }
    return operation;
}


instead of doing operation = ... and then returning operation you can just return 

Example :

case 1:
    return "+";


Same thing in the Cal method.

In the CheckAnswer method you should just have else instead of else if(userInput != sum)

Also in the Main your Random rnd = new Random(); should be out of the do while loop.