how to do the sum and average in a vector

You can create a custom extension method that will allow you to calculate the sum and average of a Vector3 in a more readable way and use it as you would use the built-in Sum and Average methods.

public static class Vector3Extensions
{
    public static float Sum(this Vector3 vector)
    {
        return vector.X + vector.Y + vector.Z;
    }
    public static float Average(this Vector3 vector)
    {
        return vector.Sum() / 3;
    }
}

Then you can use it as

Vector3 vector = new Vector3(10, 20, 30);
Console.WriteLine("The sum of the vector is: " + vector.Sum());
Console.WriteLine("The average of the vector is: " + vector.Average());

To Average vector, you can do like below. 

var newVector = new Vector3(
        list.Average(x=>x.X),
        list.Average(x=>x.Y),
        list.Average(x=>x.Z));

list.Average(x=>x.Z) will return average value of Z from all elements from list

For Vector Sum (double) type :

double vectorSum = Vector.Dot(yourDoubleVector, Vector.One);

The Dot method calculates the dot product of the two vectors, which is defined for two vectors A and B of size n as A1 * B1 + A2 * B2 + ... + An * Bn

Reference

https://stackoverflow.com/questions/33170643/finding-the-average-of-vectors-in-a-list

https://stackoverflow.com/questions/35514685/sum-of-elements-in-system-numerics-vectort-in-net-4-6

Thanks