This should be quite straight forward

private int selection = -1;

void Start()
{
    ShowNextQuestion();     
}

private void ShowNextQuestion()
{
    selection++;
    if(selection >= questions.Length - 1) 
    {
        // You said you wanted to restart the questions so simply do
        selection = 0;
    }

    question.text = questions[selection];
}

public void CheckAnswer()
{
    if (answer.text.Equas(correctAnswer[selection]))            
    {
        Debug.Log("Answer is Correct");
    }
    else
    {
        Debug.Log("Answer is not Correct");
    }

    ShowNextQuestion();
}

Let me tell you though that in general it's not a good idea to store questions and answers in two individual arrays.

• You can't really be sure that both individual arrays always have te same length
• you have double work when adding or removing a question in the middle
• Imagine later wanting to randomize the order of questions .. you lose track which answer belongs to which question

So I would suggest you rather couple them strong together in a type like

[Serializable]
public class Question
{
    // Allow to edit these in the Inspector but nowhere else
    [SerializeField] private string _text;
    [SerializeField] private string _answer;

    // Other classes may only read these
    public string Text => _text;
    public string Answer => _answer;

    // Constructor 
    public Question(string text, string answer)
    {
        _text = text;
        _answer = answer;
    }
} 

And now in your Component you rather set them via the Inspector or initialize them via

public Question[] questions = {
    new Question("What is 10+10", "20"),
    new Question("What is 20+20", "40"),
    new Question("What is 30+30", "60"),
    new Question("What is 40+40", "80"),
    new Question("What is 50+50", "100")
};

Then of course you change the code accordingly to access these

private void ShowNextQuestion()
{
    selection++;
    if(selection >= questions.Length - 1) 
    {
        // You said you wanted to restart the questions so simply do
        selection = 0;
    }

    question.text = questions[selection].Text;
}

public void CheckAnswer()
{
    if (answer.text.Equals(questions[selection].Answer))           
    {
        Debug.Log("Answer is Correct");
    }
    else
    {
        Debug.Log("Answer is not Correct");
    }

    ShowNextQuestion();
}

Now as said this allows also to give your app a bit randomness by shuffling the questions before starting:

using System.Linq;

...

private void Start()
{
    questions = questions.OrderBy(q => Random.value).ToArray();

    ShowNextQuestion();
}

private void ShowNextQuestion()
{
    selection++;
    if(selection >= questions.Length - 1) 
    {
        // You said you wanted to restart the questions so simply do
        selection = 0;
        questions = questions.OrderBy(q => Random.value).ToArray();
    }

    question.text = questions[selection].Text;
}