How to stop getting duplicate numbers while using random.range

Questions & Answers
1

I am trying to make math multiplication application for learning unity and c#. Referencing free pack from unity asset store “Math Challenge”.
Using Random.range to assign one correct answer and 3 random values to answer buttons. However, sometimes duplicate values are assigned to answer buttons. I mean same value is assigned to two or sometimes three answer buttons.I have attached screenshot.
Can any one please help me to solve this issue??
Thanks

using UnityEngine;
using UnityEngine.UI;


public class Multplication : MonoBehaviour {

    //we make this script instance
    public static Multplication instance;
    //its an enum which we help use to identify the current mode of game 
    public enum MathsType
    {
        multiplication1to10,
        multiplication11to20
    }
    //we make a variable of MathsType
    public MathsType mathsType;
    //2 private floats this are the question values a and b
    private float a, b ;
    //the variable for answer value
    [HideInInspector] public float answer;
    //varible whihc will assign ans to any one of the 4 answer button
    private float locationOfAnswer;
    //ref to the button
    public GameObject[] ansButtons;
    //ref to image symbol so player can know which operation is to be done
    public Image mathSymbolObject;
    //ref to all the symbol sprites whihc will be used in above image
    public Sprite[] mathSymbols;
    //get the tag of button 
    public string tagOfButton;
    //varible to check whihc mode is this
    private int currentMode;
    //ref to text in scene where we will assign a and b values of question
    public Text valueA , valueB;
       

    void Awake()
    {
        MakeInstance();
    }

    //method whihc make this object instance
    void MakeInstance()
    {
        if (instance == null)
        {
            instance = this;
        }
    }

    //at start we need to do few basic setups
	void Start ()
    {
        
        //we put the location value in tag of button variable
        tagOfButton = locationOfAnswer.ToString();

      
        if (GameManager.singleton != null)
        {
            //get whihc mode is selected
            currentMode = GameManager.singleton.currentMode;
        }

        //we call the methods
        CurrentMode();

        MathsProblem();

    }

    //this method keeps the track of mode 
    void CurrentMode()
    {
        if (currentMode == 4)
        {
            //depending on the currentmode value we assign the mode

            mathsType = MathsType.multiplication1to10;
        }
        else if (currentMode == 5)
        {

            mathsType = MathsType.multiplication11to20;
        }
    

    }

    // Update is called once per frame
    void Update ()
    {
        tagOfButton = locationOfAnswer.ToString();
    }


    //this methode calls the respective method for the respective mode
    public void MathsProblem()
    {

        //switch case is used to assign method
        switch (mathsType)
        {

            case (MathsType.multiplication1to10):

                Multiplication1to10();

                break;

            case (MathsType.multiplication11to20):

                Multiplication11to20();

                break;
        }
    }
       
    //this methode perform Multiplication process
    void MultiplicationMethod()
    {
        b = Random.Range(0, 10);

        locationOfAnswer = Random.Range(0, ansButtons.Length);

        answer = a * b;

        valueA.text = ("" + a).ToString();
        valueB.text = ("" + b).ToString(); ;

        mathSymbolObject.sprite = mathSymbols[0];

        for (int i = 0; i < ansButtons.Length; i++)
        {
            if (i == locationOfAnswer)
            {
                ansButtons*.GetComponentInChildren<Text>().text = "" + answer;*

}

else
{
// the below code make sure that all the values assigned to the ans button are within the range

if (a * b <= 30)
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range( 0, 30);*
}
else if (a * b <= 60 & a * b >= 31)
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range(28, 61);*
}
else if (a * b <= 90 & a * b >= 61)
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range(58, 91);*
}
else if (a * b <= 120 & a * b >= 91)
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range(88, 121);*
}
else if (a * b <= 150 & a * b >= 121)
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range(118, 151);*
}
else if (a * b <= 200 & a * b >= 151)
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range(148, 201);*
}

while (ansButtons*.GetComponentInChildren().text == “” + answer)*
{
ansButtons*.GetComponentInChildren().text = “” + Random.Range(0, 201);*
}
}

}

}

void Multiplication1to10()
{
a = Random.Range(1, 10);
MultiplicationMethod();
}

void Multiplication11to20()
{
a = Random.Range(11, 20);
MultiplicationMethod();
}

}

2

using System.Collections.Generic;

class MainClass
{
  public static void Main(string[] args)
  {
    RandNoRepeat(5, 20);
  }
  
  public static void RandNoRepeat(int min, int max) {
    List<int> vals = new List<int>();
    for (int n = min; n <= max; ++n) {
      vals.Add(n);
    }
    
    while (vals.Count > 0) {
      int index = new System.Random().Next(0, vals.Count);
      System.Console.WriteLine("-->" + vals[index]);
      vals.RemoveAt(index);
    }
  }
}