how can I get the average,,using for loop?

namespace Treehouse.CodeChallenges  //how can I get the average???
{
    class FrogStats
    {
        public static double GetAverageTongueLength(Frog[] frogs)
        {
            double toungue =0;
            for (int index = 0; index <frogs.Length; index ++)
            {
               Frog tongue= frogs[index];
               //how can I get the average???
            }
            return toungue.Average();
        }
    }
}

namespace Treehouse.CodeChallenges
{
    public class Frog
    {
        public int TongueLength { get; }

        public Frog(int tongueLength)
        {
            TongueLength = tongueLength;
        }
    }
}