Arrays - Calculating average of an array

using System;

namespace random
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] studentsGrades = new int[] { 15, 13, 8, 12, 6, 16 };
            double averageResult = GetAverage(studentsGrades);

            foreach( int grade in studentsGrades)
            {
                Console.WriteLine(grade);
            }

            Console.WriteLine($"\nThe average is: {averageResult}");
            Console.ReadLine();

            // Output:
            // "15, 13, 8, 12, 6, 16"
            // "The average is 11.66666666"

        }

        // Method to calculate average for the array of numbers.
        static double GetAverage(int[] gradesArray)
        {
            int size = gradesArray.Length;
            double average;
            int sum = 0;

            for(int i = 0; i < size; i++)
            {
                sum += gradesArray[i];
            }

            average = (double) sum / size;
            return average;
        }

    }
}