Unity c# how to check if random value was called when same function was called earlier

I am making multiplication sum where int a and int b both are getting values from random.range(1,11). and there are 4 buttons to click correct answer. when correct answer button is clicked i am calling sum creator function again so next sum will appear. My problem is some times i get same question next time. for example first time 2 X 5 and when i recall sum creator again i get 2 x 5. i tried adding value of b to a list then comparing if the value is used earlier if yes then again randomize it.( i am trying to control only duplication of b) But as per my understanding I am assigning single value to b when i call sum creator function. there is no loop because i want only one question at a time. and when i recall the same function it does not remember which value was used previously. How do i solve this? please help.

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;
using UnityEngine.UI;
using TMPro;

public class MathsAndAnswerScript : MonoBehaviour
{
public static MathsAndAnswerScript instance;
 private int a, b;
//the variable for answer value
[HideInInspector] public int answer;
//varible whihc will assign ans to any one of the 4 answer button
private int locationOfAnswer;
//ref to the button
public GameObject[] ansButtons;
//get the tag of button 
public string tagOfButton;
//ref to text in scene where we will assign a and b values of question
public TextMeshProUGUI valueA, valueB;
public static bool roundActive = false;
void Awake()
{
    MakeInstance();
}

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

void Start()
{
   tagOfButton = locationOfAnswer.ToString();
   SumCreator();
}

void Update()
{
    tagOfButton = locationOfAnswer.ToString();       
}   

void SumCreator()
{
    // we do multiplication here
    bool reloop;
    bool[] numbers = new bool[301];

    List<int> usedValues = new List<int>();
    a = Random.Range(1, 11);
    b = Random.Range(1, 11);
    while (usedValues.Contains(b))
    {
        b = Random.Range(1, 11);
    }              

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

    answer = a * b;
    numbers[answer] = true;

    if (valueA != null && valueB != null)
    {
        valueA.text = a.ToString();
        valueB.text = b.ToString();
    }
    // mathSymbolObject.sprite = mathSymbols[0];

    for (int i = 0; i < ansButtons.Length; i++)
    {
        if (i == locationOfAnswer)
        {
            ansButtons[i].GetComponentInChildren<TextMeshProUGUI>().text = "" + answer;
        }
        else
        {
            // the below code make sure that all the values assigned to the ans button are within the range

            int value = 0;
            do
            {
                reloop = false;
                if (answer <= 10)
                {
                    value = Random.Range(0, 15);
                }
                else if (answer <= 30 & answer >= 11)
                {
                    value = Random.Range(10, 45);
                }
                else if (answer <= 60 & answer >= 31)
                {
                    value = Random.Range(25, 75);
                }
                else if (answer <= 90 & answer >= 61)
                {
                    value = Random.Range(55, 105);
                }
                else if (answer <= 120 & answer >= 91)
                {
                    value = Random.Range(85, 135);
                }

                if (numbers[value]) //already select?
                {
                    reloop = true;
                }

            } while (reloop);

            numbers[value] = true;
            ansButtons[i].GetComponentInChildren<TextMeshProUGUI>().text = "" + value;
        }

    }//for loop
}

Button Script.cs

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
public class CheckButtonPress : MonoBehaviour {

private Button thisButton;

void Start()
{      
    //we get the button attached to the object 
    thisButton = GetComponent<Button>();        
}  

//method whihc help us to identify if player has pressed correct or wrong answer
public void checkTheTextofButton()
{

    if (gameObject.CompareTag( MathsAndAnswerScript.instance.tagOfButton))
    {
       // do something                       
    }
    else
    {                      
       //do something else            
    }

    //after we have answered the question we call the sum creator method to create new question
    MathsAndAnswerScript.instance.SumCreator();

}