From the instructions, it is not clear to me what should happen if maxFactor < 0.

namespace Treehouse.CodeChallenges
{
    public static class MathHelpers
    {
        public static int[][] BuildMultiplicationTable(int maxFactor)            
        {

           if (maxFactor > 0) {
                int [][] table = new int[maxFactor + 1][];                
                for (int row = 0; row < maxFactor; row++) {
                    table[row] =  new int[maxFactor +1];
                    for (int col = 0; col < maxFactor; col++) {
                        table[row][col] = row * col;
                        System.Console.Write(table[row][col]);
                    }
                    System.Console.WriteLine();

                }
                return table;

            } else return new int[0][];
        }
    }
}