How can I solve my StringBuilder problem?

Question

What I am doing: Replacing variables within a string using StringBuilder to generate questions that contain variation.

string question;

void CreateNewQuestion()
{
    Random rnd = new Random();
    int questionNumber = rnd.Next(1, 4); //Generate a random question number
    int a = rnd.Next(1, 10); //Create random numbers so the question is different each time
    int b = rnd.Next(1, 15);
    int c = rnd.Next(1, 15);

    string q = questionNumber.ToString(); 

    StringBuilder sbq = new StringBuilder("Question" +q);//StringBuilder is now called to replace the randomly called for question with its new variables
    sbq.Replace("Question1", $"What is {a} + {a} ?");
    sbq.Replace("Question2", $"What is {a} + {b} ?");
    sbq.Replace("Question3", $"What is {a*c} + {a} ?"");

    question = sbq.ToString();
}

The problem: If string q (the one being modified) = "Question1", StringBuilder.Replace won't just stop at sb.Replace("Question1"...) it will still calculate for Question 2 and 3. Thus as the number of questions increases, so does the inefficiency of this.

The question: How can I create questions that contain variables so as to provide variation within the same question structure in an efficient manner?

Answer 1

I would suggest to use Dictionary<TKey, TValue>:

 Random rnd = new Random();
 int questionNumber = rnd.Next(1, 4); //Generate a random question number
 int a = rnd.Next(1, 10); //Create random numbers so the question is different each time
 int b = rnd.Next(1, 15);
 int c = rnd.Next(1, 15);            

 var questions = new Dictionary<int, string>
 {
     { 1, "What is " + a + " + " + a + " ?" },
     { 2, "What is " + a + " + " + b + " ?" },
     { 3, "What is " + (a * b) + " + " + c + " ?" },
 };

 var question = string.Empty;
 if (questions.TryGetValue(questionNumber, out question))
 {
     // key exists and you can read your question here
 }

UPDATE:

If you want to have a dictionary to be created in one function and the random numbers in another, then you it is necessary to have one goal functions:

static Random rnd = new Random();
static void Main(string[] args)
{   
    int questionQuantity = 15;
    var questions = new Dictionary<int, string>();

    for (int i = 0; i < questionQuantity; i++)
    {
        int variableCount = rnd.Next(17);
        var variables = CreateVariables(variableCount);
        var signs = CreateSigns(variableCount - 1);
        var question = CreateQuestion(variables, signs);
        questions.Add(i, question);
    }  
}

And other functions:

public static List<int> CreateVariables(int variableQuantity)
{
    var variables = new List<int>();
    for (int i = 0; i < variableQuantity; i++)
    {
         variables.Add(rnd.Next(1, 15));
    }
    return variables;
}

public static List<char> CreateSigns(int variableQuantity)
{
    var operators = new char[] {'+', '-', '/', '*' };
    var randomOperators = new List<char>();
    for (int i = 0; i < variableQuantity; i++)
    {
        randomOperators.Add(operators[ rnd.Next(0, 3) ]);
    }
    return randomOperators;
}

public static string CreateQuestion(List<int> variables, List<char> operations)
{   
    StringBuilder sb = new StringBuilder();
    sb.Append("What is ");
    for (int i = 0, j = 0; i < variables.Count; i++)
    {
        sb.Append(variables[i]);                
        if (i % 2 != 0)
        {
            sb.Append(" ");
            sb.Append(operations[j]);
            sb.Append(" ");
            j++;
        }   
    }
    sb.Append("?");
    return sb.ToString();
}