using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string text = "The quick brown fox jumps over the lazy dog.";

        string firstSymbol = "<b>";

        string secondSymbol = "</b>";

        List<int[]> arr = new List<int[]>

        {

            new[]{35, 8},

            new[]{4, 15},

        };

        arr = arr

            .OrderBy(x => x[0])

            .ToList();

        int previousIndex = 0;

        foreach (var value in arr)

        {

            int AddedLenghtForNextIndex = value[0] + previousIndex;

            string textValue = firstSymbol + text.Substring(AddedLenghtForNextIndex, value[1]) + secondSymbol;

            previousIndex += firstSymbol.Length + secondSymbol.Length;

            text = text

                .Remove(AddedLenghtForNextIndex, value[1])

                .Insert(AddedLenghtForNextIndex, textValue);

        }

​

        Console.WriteLine(text);

    }

}