using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var array = new [,]

            {

                {1, 2, 3, 4, 5},

                {6, 7, 8, 9, 10},

                {11, 12, 13, 14, 15},

                {16, 17, 18, 19, 20},

            };

        List<int> row_sum = new List<int>();

        for(var rowIndex = 0; rowIndex < 4; rowIndex++)

        {

            var sum = GetSumOfRow(array, rowIndex);

            row_sum.Add(sum);

        }

        Console.WriteLine(string.Join(Environment.NewLine, row_sum));

    }

     public static int GetSumOfColumn(int[,] matrix, int columnNumber)

    {

        return Enumerable.Range(0, matrix.GetLength(0))

                .Select(x => matrix[x, columnNumber])

                .Sum();

    }

​

    public static int GetSumOfRow(int[,] matrix, int rowNumber)

    {

        return Enumerable.Range(0, matrix.GetLength(1))

                .Select(x => matrix[rowNumber, x])

                .Sum();

    }

}